Things that spring to mind (assumes a few specific bits I'm not sure whether are taught at high school, because I was home ed and skipped most of school):

• Cowpox vaccine - A safe, readily available, protection from a major threat. May be hard to convince people to use it, but if you spread the idea and take credit it'd catapult you into fame (particularly medical).
• Penicillin (would need to test a load of different moulds to find the right one), germ theory, sterilization, hygiene, other medical basics. - Once you've got a name for yourself, start spreading other modern medical standards.
• Depending on exact time-period, I may be able to preempt gunpowder. This may set me up nicely with whichever king is in charge.
• Attempt to build basics of electricity. If you can find a magnet (magnetite?), making a basic dynamo is not too hard, and would be an awesome way to show off your knowledge (though may get you burned as a witch).
• My everyday clothes would be really strange to someone from that era. Hide them until I find people who seem trustworthy enough/of the right charachter, build a secret society/cult with them as sacred objects, with top members knowing some of my actual story (avoid conflict with church/royals, perhaps claim I'd been sent back by god?). Find students, teach them everything I can remember (perhaps bar a few things like nuclear weapons), spend most of my life organization-building and laying plans for the future. Give them a mission, to take the knowledge around the world, and avoid some of the largest disasters coming.

Organization goals:

• Reach the Americas with the cowpox vaccine before smallpox (even if it takes 100s of years before ships go out, make sure my cult is leading them).
• Intercept every super famous person I can remember as they come into adulthood, attempt to bring them into the group, give them a list of topics I remember them having made major progress on, give them all I remember of their results.
• Be hyper-secretive about the core books on the future (especially historical events and non-medical technology), and the fact that this is what's happening, unless very large gains can be made.
• Spread certain moral ideas, when possible. Have locked books which are only to be opened when specific criteria are met (e.g. to trigger dropping internal religious elements when it's safe). Maybe a very simple cipher which splits information across several books, so locked books can be copied without being read.

Set in motion plans for dealing with current issues

• preempt global warming by introducing photo-voltaic early (even if I don't know details, I can point at roughly the things needed and someone will figure it out), may not be perfect or win out right away but hopefully sooner than the normal timeline with the right talent
• attempt to shut down nuclear weapons, even if this requires major deception. Backup plan, gain a monopoly of them.
• Set FAI work in motion much earlier, with the power of a ~700 year old conspiracy that's had foreknowledge of every major technological advance and captured a notable fraction of world-changing geniuses.

All a bit optimistic, but it seems like the vast majority of the good I could do would happen after my lifetime, so I'd need to take some shot at making a lasting organization. If it seems plausible, and I'm in a position by that time to direct people and live where I want, try to die somewhere near a place I could get natural cryo.