First wait and see what the school does decide to do. Follow up and find out if the school contacted the girl's parents. If possible, see if you could meet with the girl's parents and the children involved and discuss the seriousness of the issue.
You did the correct thing by taking his phone away. When you decide to return it, maybe have some features blocked so that he can only make calls and not receive text messages and/or pictures.
Keep a close tab on who all or most of his friends are and the whereabouts of your son at all times.Be open and honest with your son about just having lots of friends as opposed to having a girlfriend and that there is no need to rush into a male/female relationship. Nowadays, it is the girls who are pressuring the boys into doing certain things that are not appropriate.
Always tell your son that you love him and that you support him. However as his parent, it is your duty to let him know when things are unacceptable.
You can also not allow him to hang out with friends or play video games if he likes those things. But keep him away from the girl as much as you can. It may be hard if they attend the same school though.
I have 3 sons so I understand what you are going through. Your son must realize that there are negative consequences for misbehavior. Do not let him have the upper hand. Always remain in charge as the parent and use your assertive leadership skills to deal with your son.