I’d advise to stick with MonoGame because it supports all current platforms: Windows, Mac, Linux, iOS, Android, UWP, Web, Xbox One, Nintendo Switch and PS4. Next gen consoles in the near future. (PS5 is already in the works: https://twitter.com/monogameteam/status/1278419015262748672). Also setup and run a cross-platform project is very straight forward (Made a little tutorial here, btw: MonoGame development on macOS)
From all platforms above, the ones SDL2 lack are PS4 and Xbox One support. Don’t know if they will ever add them or even support next-gen.
I wouldn’t consider learning a different language than that your teacher is currently teaching you guys a “small problem”. More like a very interesting experience as you can perfectly learn both at the same time. I myself find C# easier to work than C++.
Well, whatever your choose will be, I wish you luck 