For your school email - apparently it depends on your IT administrator. Some might delete your accounts. Some might let them stay. According to some, your school email is meant to be deleted 3 months after graduation - though I can't verify that figure.
In any case, do back up data just in case your email is deleted.
On a similar vein though, schools will keep your data about how you went in the HSC...
if you got an independent/private school (your email wont have the @education.nsw.gov.au tag) they delete it completely pretty soon after HSC and for public it is as c_z_m says where it is deleted roughly 3 months later.
If you removed it, you'll probably have to ask for permission again to be added back. If you haven't, it should be visible on your "Shared with me" Drive page. I'm not sure what your circumstances are, but if all else fails, it won't hurt to ask the original owner!