chakka, Chambers online dictionary says
"exclude verb (excluded, excluding) 1 to prevent someone from sharing or taking part. 2 to shut someone or something out, or to keep them out."
"expel verb (expelled, expelling) 1 to dismiss from or deprive someone of membership of (a club or school, etc), usually permanently as punishment for misconduct. 2 to get rid of something; to force it out."
If a pupil has been excluded, have they not been prevented from taking part in the school (temporarily)?
If a pupil has been expelled have they not been dismissed permanently?