Hello- Thank you for asking the question. I have over 30 years of experience working with individuals, couples and families & am happy to reply.
I am sorry to hear about this situation.
I have worked with children for over 30 years and I can tell you conclusively that this NORMAL behavior however, your daughter should understand that this kissing boys at school is Not appropriate.
Sit her down and normalize the curiosity aspect of this and that although she is curious, the behavior can and will cause problems at school. I think this will nip it in the bud.
As I said, this is not atypical at all for a child of this age and don't allow the school to tell you otherwise.
If everything else in her life is in balance, she is just challenging curiosity and this trait when channeled in the right direction can lead to all kinds of good things. Teach her the benefit of positive curiosity.
The following link explains this in helpful detail.
Should you have additional questions of me, feel free to reply.
Otherwise, I wish you, your daughter and family.......the very best.
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Thanks & have a great day.