Good Morning L.! First take a Deep breath, get your thoughts together (write them down if you need to), then head for the School. Or call and make an appointment for a conference with the principal and the teacher at the same time. Tell them it's very important that you speak with them Today! Not next week, or two days from now.
Little kids can be real buggers at school, especially little boys who think it is fun to tease little girls. Not picking on boys Mama's I raised 2 and have 4 gr son's.
For the little guy who is supposed to be her Boy friend they don't understand what that means at 6. Maybe your daughter is old enough to say something like "I'm your friend and that was Ugly to say to me". "I'm still your friend so don't say bad things anymore". It used to be the more terrible things a little boy said to a little girl the more he liked her but didn't want anyone to know!
As for the Kissing and removing cloths OMGoodness! He has seen something he really should not of seen somewhere!!
After speaking with the principal I would suggest they call this child's parents and ask them to come in. They need to know what was said to your little girl. Maybe he saw a Magazine or movie he shouldn't of. But they definitely need to know about it. I don't think schools can speak to a child about something like this without the parents being there. Children by nature are innocent (used to be anyway), having NO idea what this is all about.
If we face it with a cool head, non accusatory fashion we might get better results. Instead of angry and demanding.
Don't wait L., do it today.
God Bless and keep breathing, helps keeps the anger at bay.
K. Nana of 5