Tutoring someone for free makes you twice as smart: once for learning it, once for explaining it.

Explaining something forces you to find the places where your own understanding is vague. You can follow a method and still not know why it works, and that gap stays hidden until somebody asks. Teaching drags it into the light. The other person gets help and you get a map of your own blank spots.

Offer an hour this week to whoever is behind in a class you are fine in. Do not prepare much. Let their questions show you what you only half know, then go look those parts up before the next test.