Khan Academy
Especially good for math and computing for all ages but other subjects at Secondary level. Note this uses the U.S. grade system but it’s mostly common material.

Futurelearn
Free to access 100s of courses, only pay to upgrade if you need a certificate in your name (own account from age 14+ but younger learners can use a parent account).

Openlearn
Free taster courses aimed at those considering Open University but everyone can access it. Adult level, but some e.g. nature and environment courses could well be of interest to young people.

Crash Course
You Tube videos on many subjects

Crash Course Kids
As above for a younger audience

Prodigy Math
Good for elementary school ages

Learn Computer Science