In my Catholic high school, there was at least one non-Catholic Christian in our class of about 120 kids, but he fit right in. The only thing that might make him feel uncomfortable are the times when they go to Mass, he wouldn’t be allowed to receive the Eucharist and may feel a little left out because of that, but other than that, I’ve found kids to be very accepting. Though, cliques, popular kids, and bullies exist in all schools, though I feel in a Catholic school bullying would be less tolerated than they are in public schools.