Khan Academy

A teenager stuck on quadratics the night before a test

Nobody at home can help, the textbook explanation has already failed twice, and the alternative is arriving tomorrow having not understood it. Khan Academy was built for exactly that evening, and it gives the whole thing away: no advertising, no paywall, no account needed to start. Adults filling gaps they never filled at school are the other half of its audience, and a lot of its use happens quietly through classrooms rather than through anyone choosing it.

Practice first, video second

The structure is mastery-based. Watch a short explanation, then work problems until the idea actually holds, rather than watching an hour of video and mistaking recognition for understanding. Progress is tracked per topic, so what you’ve actually grasped is visible next to what you only think you have. Hints arrive mid-problem, which keeps a stuck student moving instead of closing the app.

The maths runs in proper sequence from early arithmetic through to university calculus, and the science tracks follow a similar shape. For a student who is behind, the sequencing is the whole gift: they can start where their gaps actually begin rather than where their year group happens to be, and nobody has to see them do it.

Strong in maths, patchy elsewhere

Step outside maths and science and the quality drops off, which is the honest limit on this one. The humanities and arts material is thinner and less structured, closer to a collection of resources than a course, so a student who arrives expecting the same guided path through history that they got through algebra will find themselves wandering and will probably blame themselves for it. The mobile interface also shows its age next to newer educational apps, and the exercise screens weren’t designed for thumbs first, so entering an answer is fiddlier than it needs to be. There’s no real starting point offered to someone who doesn’t know what to pick either.

For arithmetic through calculus, and for the science alongside it, this stands with the best free teaching available anywhere. Point a struggling student at a specific topic rather than the front door, treat the humanities material as a bonus, and it will do more for them than most things they pay for.