effects of crime and violence on students

[escalade]

how can crime affect a student's school life?

[Beria]

If someone murders them they won't be able to sit their final exams.

[davec886]

Thanks Beria for a totally useless answer!
If you mean being a victim of crime then this can be very psychologically damaging and can be very detrimental to study.
Being an offender can be problematic as many schools are part of safer schools partnerships which involve local Police. Schools have a duty of care to their students and if a student is violent, a drug dealer, a sexual predator or other wise a danger to other students they may well find that they are excluded. Such information can lawfully shared between Police and schools under s.117 of the Crime and Disorder Act. Without more info it's difficult to answer your question more fully.