They came to visit. Background music can be very conducive to visiting and it can be a lot cheaper. Check costs of bar tenders and cost of drinks. Consider an all-day event with constant supply of food. This allows more visiting.
Name badges are very important.
Try to pre-print them so that they are legible. Invite Teachers and Staff Teachers love to come to these events, don't be discriminatory - try to invite all of your teachers, principals, guidance conselors, nurse, etc. If you expect more than 100 classmates plus guests consider having an off-duty policeman or security guard. This usually is not very expensive. Rewiew Afterwards, conduct a review of the event within one week while things are still fresh.