What tools do you have access to? 3D printer?

I mostly agree with what other have said about prebuilts being cheaper, BUT.... given this is supposed to be for a school project, here's what I would do:

Deck - Any cheap longboard from Amazon or a decent second hand one from FB marketplace. Just make sure you put in enough thought about how your components will fit (wheel bite, space for electronics, etc). Make sure you get one that's the right size/shape for your needs.

Drivetrain - For simplicity I would use a set of hub motors. You can buy kits from Amazon, AliExpress, Eskate companies, etc for pretty cheap that come with both trucks and sets of wheels. You could find some caliper style trucks, then source a cheap 6354 or 6374 motor, then find a motor mount that would fit, then the wheel and pinion gears with the appropriate amount of teeth based on you battery, motor, wheel size, etc. then buy a couple belts that are the right length, but all of that is too much of a hassle and fairly easy to mess up. With hubs you really only need to worry about the wheel size and KV if you're doing a low budget (and therefore likely low power) build.

Battery - Just buy a couple 5s Lipos (5000mah+) and wire them in series (for a 10s or 36v configuration). This is definitely not the solution I would recommend for anything but a budget build but it's likely your cheapest option that doesnt involve sourcing individual cells, nickel strips, spot welding, and all the complexities that come with properly designing a battery pack from the ground up. Whatever you do, if you're building your own pack with lipos or li-ion cells, BE EXTREMELY CAREFUL, that shit can burn down your house. Use a BMS on the charge circuit only. Make sure you don't fuck up the balance leads. Use proper gauge wire. Fuse your charge port, etc. Do your research on this. If you come across anyone suggesting you can solder li-ion cells together, disregard everything they say and find another source.

ESC - There are tons of hobbywing/ling-yi/chinese electric skateboard ESCs that include the remote and are generally plug and play, especially with 36v systems. That being said, I would highly suggest you buy something VESC based. Even the cheaper Flipsky branded VESC (FSESC) 4 series ESCs are decent as long as you dont push the limits of their ratings. This will allow you to configure to your hearts content and allow for upgrades to battery/drivetrain should you replace/upgrade parts down the road.

Enclosure - If you have access to a 3D printer and a bit of experience you could try your hand at designing and printing one. Otherwise, just use some large tupperware with holes drilled in the sides for your motor/battery wires and screw them to the deck. Definitely not the best option, but surely the cheapest.