L., I just wanted to be in touch and say I'm sorry that this is added to the mix for your daughter. I had replied to the earlier e-mails and was so happy to hear that the teacher had been so great, and now -- this. As others have said, this is not just bullyiing but sexual harassment.
You are right to have your older son there with her for a time, but eventually he won't be able to do that daily. The assistant principal said she'd handle it, but how? I am curious exactly what steps she plans to take, and I would advise you to ask her for the specific steps -- which ought to include her talking in person to the parents of the one boy who is so badly harassing and following her (and not just her sending them a note or e-mail). She also ought to talk to whatever teacher is in charge of the student patrols; usually there is a teacher "sponsor" or "advisor" for patrols, and that person also should talk directly to the boy and/or the parents.
Follow up; ensure that these talks actually do take place, and take place in a certain time frame, so that the principal and patrol advisor can't let weeks pass by saying "Oh, we haven't been able to reach those parents yet...." It sounds like the school is being responsive but I would definitely ensure they know you will keep tabs on them until you know the parents have been spoken with and the boy put on notice.
Frankly I'd like to see the boy given a one-day suspension for this with notice he will serve a much longer one if he does it again. This behavior does not seem like normal teasing, bullying or harassment; it seems to have stalkerish, sexual overtones (becaues of the the "she likes lots of boys" origins of it) and that is worrisome.
As for your daughter, you already know the drill: She has to learn not to react because any reaction, whether it's anger or tears or anything in between, fuels this fire.These kids need to tire of it, which is hard on her. Of course the other alternative is that the one kid who is following her might get a real shock if she turns on him and screams down the neighborhood (not at school) that "This kid is following me and harassing me and I want someone to call the police RIGHT NOW!" I hesitate to advocate that, not knowing how he might react; some bullies back down instantly when confronted, especially by someone they think is weak and won't talk back, but others could get worse. And as others noted - if you have to tell the school and the boy's parents that you will call the cops next time he follows and catcalls at her, do so. Then really do call them if it happens. You and your daughter will not be popular for that, but it might finally put some fear into the whole pack of boys.
Be aware of one thiing: If he loses his patrol spot because of this he may make your daughter's life even worse for a while and you need to warn her, if that happens, to hold up her head and not put up with any, um, junk over that. He deserves to lose his spot and if that happens it may convince other boys to stop.
Have you also talked with her classroom teacher and let her know all that is going on, that you have talked with the principal, etc.? I think her added support just being attentive to your daughter right now could be helpful, even though she can do nothing about the stalker-boy. If the other boys who are bullying your child are in the same class, though, I would expect the teacher to help deal with them, at a minimum by talking to the entire class (with the school counselor there) about gossip and bullying. (If that happens, your daughter should NOT be made the example! It would be horrid if it came out as "We're all here to talk about Sally and these boys..." Any good counselor and teacher should know better but ensure they know not to make a class counseling lesson about one incident or one child!)
Please keep us posted.