How about swimming?
I know I had massive hip and knee pain(nothing officially by doctor, but it hurt like hell) in the beginning of summer in June. I mostly swam, and maybe one land exercise day a week, then one walk a week. I gradually built back up to intense land exercise.
In the pool, I find that I can still do pretty vigorous things, and have things hurt 1000% less(zero or almost zero). Ask your doctor if you can swim, see if your school has a pool?
If it still hurts the knee swimming, try swimming without using legs(squeeze one of those little tools that keeps legs together). Now THAT'S intense!
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