Sorojos he says that they will see it.
"Can my school see which sites I visit if I use VPN"
If "it" refers to "which sites" (rather than see all traffic), then @doffactory had the correct answer. If your DNS requests are not properly routed through VPN, then the school can see which sites (domain) but not which page or what is being read or submitted in any forms. For example, they could see www.youtube.com, but not which video.
DNS should be tunneled by default for most VPN apps I believe. Verify that your VPN is tunneling DNS requests using the app settings and confirm using dnsleaktest.com.
If you need to explain to this guy, then first learn about DNS. I prefer the Phonebook analogy for beginners.