Why your child forgets what they were taught in 11 plus maths

There is a sentence almost every parent has said over a maths book, in almost exactly these words.

"Come on. You know this. I know you know this. I taught you this myself."

Sometimes with a laugh. Sometimes through gritted teeth at half past eight on a Tuesday. And the frustrating part is that you are not wrong. You did see them do it. It was in an exercise book six weeks ago, three ticks in a row.

So where did it go?

Three things have to happen. Each one leaks.

For a child to still know something in an exam, three separate things must go right. Not one. Three.

1. It has to land when it is taught.
2. It has to stay in the weeks afterwards.
3. Somebody has to check, at some point, that it is still there.

Most conversations about a struggling child assume all three happened and the child is the problem. Usually one of the three quietly failed, and nobody was watching the right one.

One: not everything taught lands

A teacher stands in front of thirty children and explains a method once. It is a good explanation. It works for most of the room.

But an explanation that clicks for twenty-five children will not click for all thirty — not because the other five are slower, but because an explanation is a route, and different children need to come at the same idea from different directions. The child who needs the second route does not usually know to ask for it. They know the room nodded, so they nod.

Then the lesson moves on, because it has to. A syllabus is a timetable, and the timetable does not pause for one child who half-followed step three.

So a child can be present, attentive, perfectly well behaved — and still walk out with a small hole in the middle of a method. Not a missing topic. A missing step. That is where knowledge gaps in maths actually begin: not with a lesson missed, but with a lesson attended.

Two: what lands does not all stay

This part is not a failing. It is how memory works, for everyone, at every age.

We have known the shape of it since the 1880s, when a German researcher spent years testing his own recall and plotted what he found. It has been repeated many times since, most recently in a careful replication a decade ago. Every version produces the same picture: a steep drop soon after learning, then a long slow fade. It has a name — the forgetting curve — and it is the most reliable thing in this whole article.

Notice where the curve is steepest. It is not months later. It is almost immediately. Most of what is going to be lost is lost early — while the exercise book still looks full of ticks, and while everybody involved still believes the job is done.

Three: what gets revisited is not everything

Good teaching does come back round. Topics are revisited, past papers are set, the class goes over it again before a test.

But revisiting a topic is not the same as checking every part of it. A revision lesson on fractions covers the headline skills — the ones that come up most, the ones the class as a whole found hard. The narrow sub-steps, the awkward corners, the one method your child specifically half-followed back in Year 4: those are not on anybody's list, because nobody knows they need to be.

And "the class went over it" is not the same as "this child still has it."

The trap: everyone is certain, so nobody looks

Here is the part that does the real damage.

The teacher knows they taught it. It is in their planning. It happened.

You know they learned it. You watched them do it at the table. That happened too.

Both of you are telling the truth. And because you are both certain, neither of you thinks to check. The certainty is not a small factor — the certainty is the mechanism. It is precisely because everyone is sure the knowledge is there that nobody ever goes and looks.

And the child agrees with you

So you say it. "Come on, you know this."

Now think about what you have just asked them to do. In front of you, they have to either admit they do not know something you have just said they know — or nod.

They nod. Most children nod.

That is not lying and it is not laziness. It is the easier of two uncomfortable options, chosen quickly, by a tired ten-year-old. And often they are not even being evasive: they half-remember it. They recognise the words. Recognising something feels almost exactly like knowing it, right up until you have to produce it on your own with a clock running.

So the moment passes. The gap goes underground. And now nobody in the building knows it exists.

Where it finally shows up

Months later — in an 11+ paper, a SATs paper, an end-of-year test — it comes out as a wrong answer that looks like nothing much. A slip. A silly mistake. The sort of thing that gets the advice slow down, read the question, check your work — none of which can help, because the child was not careless. They did not have the thing.

That is the cruellest part of it. The gap arrives wearing a disguise, and the disguise sends everybody off in the wrong direction.

You cannot find it by asking

The instinct is to ask. "Are you OK with fractions?"

They will say yes. And they will mean it, because they can picture the lesson, they remember the ticks, and the word "fractions" feels familiar. Familiarity is not knowledge, but from the inside it is almost impossible to tell them apart. This is true of adults too — it is why re-reading a page feels so much more productive than covering it up and trying to write it out.

Asking a child what they know is asking them to measure the one thing they have no way of measuring.

What actually finds it

The only thing that reliably finds a gap is a question the child cannot get right by half-remembering.

Not one question — a topic is not one skill, it is dozens of small ones, and a gap lives in exactly one of them. Ask three questions about fractions and you have tested three of those small skills. If the hole is in a fourth, your child answers all three correctly, everyone relaxes, and the hole is still there.

That is why each topic gets a paper of its own rather than one broad paper skimming everything. And why every mistake on it is asked more than once, in a different form every time. Not to be exhausting — so that a right answer actually means something. Every question is built so that one particular misunderstanding, and only that one, produces a wrong answer. When it comes back wrong, we know why. And when it comes back wrong more than once, that is not a slip. That is the gap, finally visible.

Then the fix is narrow and dull and effective: practice on that exact thing, again, until it holds. Not the whole topic again. Not another practice paper. The specific step that went missing two years ago.

The thing worth taking away

Your child is not being difficult, and they are not going backwards. Something was taught, some of it did not land, more of it faded, and nobody checked — because everybody was sure.

Forgetting is not the failure. Not knowing which parts were forgotten is the failure, and it is a completely fixable one.

I have used fractions as the example all the way through, but almost nothing here is really about fractions. The same three leaks happen in place value, in long division, in decimals, in times tables, in angles, in ratio, in reading a timetable — in anything that gets taught once, practised for a fortnight, and then quietly assumed for the next two years. Every topic is made of small steps, and every one of those steps is somewhere a gap can sit.

That is why there is a separate paper for each topic rather than one that skims all of them: twenty topics, one paper each, so that whichever one is the problem can actually be looked at properly.

If you already have a hunch about which topic it is, start there. If you do not — and most parents do not, because the gap has spent months disguised as carelessness — start with the one your child would least like to be tested on tomorrow. They usually know, even when they cannot say why.

The next time you hear yourself say "but I taught you this" — you are almost certainly right. That is exactly what makes it worth going and looking.