Interesting suggestions in this thread but there are some things I'd like to point out that might throw a wrench into a lot of people's plans:
even with Highschool knowledge youll be one of the smartest.
You wouldn't. You might know a few more tidbits about the current world but you wouldn't be smarter than them. In fact, if this is high medieval times you and I might be below average when it comes to IQ. Most of what you learn in highschool that is applicable, like maths, language, physics, nature, and so on, they already knew. The things they didn't know back then hardly get touched on in highschool and not at all in such a way that you might use that extra knowledge to create something better. Most likely you'd come across as someone who pretends to be smart by saying a bunch of non-sensical stuff but when you're questioned about deep philosophical and logical questions, you'd appear as quite below the intelligence of one of their higher thinkers.
With the amount we as people have developed as a whole, surely it'd be pretty easy to do many things.
That would depend on whether you could recreate those things yourself. The concept of electricity is alien to them so explaining it to them is useless, they have no concept of it to even grasp. Unless you can create all the parts of whatever engine, device or concept you have in mind and you can prove its practical advantages, any partial knowledge of how modern devices work is useless. Even the most highy skilled electrician could not make an iPhone from complete "elements pulled from the earth" scratch, for instance.
my mathematical skills could open many doors for me,
I am not naive either so the typical manipulation tricks such as flattery and shit like that that were frequent would not work on me, I also have an understanding of capitalism, based on investments and good decisions I could generate good income to forge a name and obtain power through economic control.
In that era and especially in medieval fantasies there are many species looked down on and slave markets,
Just your mathematical skills would likely be on par at best with their best. An understanding of capitalism is useless in a feudal system. If they do have capitalism, they likely do not have slavery. If the medieval setting is high European, they also likely do not have slavery. You might apply your knowledge of capitalism at a rudimentary degree, like a money lender, but they had those in medieval times as well. You'd be at no particular advantage compared to them.
I could get steam power going, but I'd need a blacksmith to make a boiler and other parts. It wouldn't be very powerful, but I would think I could find a use for it. A fire underneath a mostly enclosed boiler filled with water, a pipe coming out of the top connected to something that spins or rotates or a lever.
Also, thinking about genetics with farming/harvesting. Taking the best of whatever food it is to seed the next generation. It wouldn't be fast, but it can be done. (This is remembered from high school biology, I have no actual practical experience doing this.) I have no idea if they had any knowledge of this or not.
Washing hands/hygiene. Using clean cloth for bandages.
Those are just some things off the top of my head.
A steam engine would depend on which medieval era (it's 1000 years after all) we're speaking of. Unless it's high medieval time, metallurgy wasn't advanced enough to produce a boiler, cylinder and shaft of sufficient tensile strength. Even if it could be theoretically done, the price for the materials would be prohibitive for your parents.
By 1200 AD, we're already 5500 years into the agricultural revolution. No amount of selective breeding you do (which they already did and knew about) will produce a stronger crop than what they already have. In fact, without GMO technology, any experiment you do is as likely to succeed or fail as they do, and it would take many generations of trial and error and plain luck to grow a stronger crop.
They already washed their hands, had soap and used clean, boiled bandages. Unless you know specific mechanical medical procedures, like how to clamp an artery, you're not bringing anything to the table that they didn't already know.
I'd join the business world and introduce them to capital, shares, corporate structures, investing, and double entry accounting. Then could move on to teach the noble houses about risk management strategies. Would make bank and be able to enjoy my free time with some of those expensive slaves. Who knows, maybe even live a sappy love story with one where I set them free and we live together.
Good luck convincing a feudal lord to give you shares for money on a harvest he already owns by hereditary right. You cannot buy land if you're not born as gentry. You might start a rudimentary stock investment program, similar to what the Dutch did with cargo ship hauls in the 1600s. There would need to be something like a free city or hanzestad that is particularly mercantile. If this is high medieval time, they might have already invented such a system, and you bring nothing new to the table. Modern corporate structures would most likely simply not be recognised by contemporary authorities. The idea of a legal entity that can act as if it were a natural person and that the legal entity and not the owner bears responsibility and is taxeable is completely alien to them.