I was doingy student teaching the first time o was pregnant. If they were polite about it, I explained that I dont really like by belly touched. If they were rude, I reached put and put my hand on their bellies. But that was for older Ida. For younger kids, I pretty much just let them and explained to them about it. I also explained that they needed to always ask first before touching. I never really had a baby shower, so I don't know.
I am also a HS teacher. I teach seniors almost exclusively and it is a development and parenting class. Because they learn information in the class that makes them curious about pregnancy, I let them feel the baby if they ask and I feel up to it. As far as a shower, I would invite all so no one feels left out, but word it carefully so they don't feel obligated. We throw diaper showers after the baby is born at my school, so I'm not sure how to approach that really. Maybe you could do something like that. It's drop in, very informal, and people can feel included without feeling awkward if they don't know any of your friends or family.
Maybe post an invite somewhere, like in the teachers lounge, so they are all invited but don't feel obligated? And ask a close coworker/friend to be the contact person to RSVP to.
As a high school student that was invited to my freshman teacher's baby shower/bridal shower it was a student who threw it for the teacher and put a sign up for anyone who wanted to go and that everyone was welcome. It was cute and not alot of students went and those who did knew the teacher and had kids of their own or on their way to gettin their own lol but yea it was in after school like a 2hour thing it was cute. Good luck and btw this was last semester that they did this.