My school is doing one of those fundraiser a where you sell things. I was thinking I could make some extra money by marking up the prices, selling them at that price, then buying them at the normal price. I'm thinking that could be illegal somehow but I don't know enough about laws to tell.
Reselling products is legal - its called "business".
What you are proposing to do is illegal - its called "fraud". Disguising personal gain under a charitable guise is fraud, a crime against the state for which fines and prison are the penalty and also the civil tort of passing off against the school for which they can seek recompense.
If you were open and honest in your sales pitch - "$5, $1 for me and $4 for the school" then this would be legal.
Generally speaking, you are committing fraud, by using the imprimatur of your school's fundraiser to take personal profit.
Regardless of the legality, does that idea not set off your "this idea is a stinker" radar? People are buying that worthless, useless stuff because it's for a CAUSE. They could otherwise go to walmart and buy that same worthless, useless stuff for much less. You, adding on a "premium" that will go in your pocket, is (let me find the correct legal term...) a "scumbag" maneuver.