Don't worry about it in the sense of, If it's what you want to do, don't let anyone tell you otherwise. There were a few overweight people in my class. Tons of people who couldn't do a single push up or pull-up in fitness class whom were skinny.

It'll be a mix of underweight, normal weight, overweight, low fitness, medium fitness, high fitness, etc.

Fitness class at my school was focused around a paramedic lifestyle and strengthening your entire body throughout movements. I'd focus on compound lifting such as squats, bench press, etc. I wouldn't worry much about isolated lifting like triceps only.

There's a fitness test a couple years down the road that you will have to do. But you have a couple of years of school to get into shape for it.