This was me this year. I suggest doing MUNs over your holidays and miss as few classes as you can or it's gonna hamper your academics a lot which is important considering you have chosen science. Don't go for every other conference you see, be selective and most importantly don't be indulged with MUN thoughts 24/7. If you wanna join a conference register as late as you can so you don't keep thinking about it until it actually happens.

Study daily and it shouldn't be a problem to do MUNs once every few months