If the behavior is restricted to kissing and hand holding, then I believe your instincts about it being just a phase are most likely to be correct. It is normal behavior for children this age to be curious about kissing and to be imitating adult or teen behavior. Behaviors like this can spread through groups of children like a new fashion trend, and fade just as quickly. If the other girls are egging her on, or if she finds that she can get lots of attention (both positive or negative), then the behavior is likely to persist until she loses interest in it.
I am relieved that you mentioned how careful you have been about exposing her to adult media. As you know, many young girls are exposed to television shows, advertisements and marketing that encourages them to try to be sexually attractive far too soon. You may notice when you go into trendy girl fashion stores that much of the clothing is more suitable for teens (or prostitutes!) and that these stores are even selling underwear that looks like lingerie. It is so hard to protect your daughter from absorbing these messages, even with all the precautions you have taken.
I would propose that you listen to the school's ideas about how to manage these behaviors and see if their plan makes sense to you. You do not have to agree with it if you believe they are planning to be too punitive. Keep in mind that attention and natural curiosity are most likely fueling these behaviors, so the consequences should be mild. It is not appropriate for anyone at school to shame her or give harsh punishments (missing recess for a week etc.). The best way to manage this is probably to make sure she and the boys involved are more closely supervised, particularly during times of the day like transitions, recess, locker time, or at the bus stop. If increased adult supervision does not discourage her, she may need simple consequences such as having to miss the remainder of recess after an incident occurs. Ideally, a staff member would pull her aside and quietly direct her to go to the guidance office or library for the remainder or recess or until she feels that she can follow the playground rules again.
At home, you can let her know that kissing is a very nice thing, but that there is a time and a place for everything. Let her know that there are lots of loving kisses between parents and children or sometimes between friends, but that boy-girl kissing is something for when she is much older. You can also talk with her about finding ways to play with boys that are ok, such as developing her soccer skills.
Finally, if the behavior persists despite everyone's best efforts, a psychologist can help. Your pediatrician should be able to provide a referral to an appropriate psychologist.
Disclaimer: This post was written for informational purposes only. It is never meant to replace face to face psychological or medical care. This post is not intended to create a patient-clinician relationship, nor to give or rule-out a diagnosis.
Try to explain her about her behavior and its draw backs in a indirect way.