Need advice: should I tell my boss that I am TTC?

I have now received a second phone call from my former lovely boss offering me a job at my old school.

I'm a teacher and was transferred to a new school this year. It was very sad to leave and they clearly felt the same.

My current job is obviously not very physical. It's classroom based SPED. The new job is that of a para which is a tough role. I'm torn because I love and miss that school, but I don't think I would be able to do the job well if I become pregnant. I worry about my physical abilities (I'm also plus size and the job involves lifting kids) and my safety (some of the kids kick and hit). If I don't end up pregnant, it would be fine.

Obviously the school year is starting soon and I don't know how to respond. I'm not pregnant yet but feel the need to prematurely turn down the job. Can you ladies help me figure how what to say?