I know others are saying, "It's normal" -- but while curiosity and even playing doctor are normal, it is NOT normal or acceptable for it to happen repeatedly with the same boy AT SCHOOL where they supposedly are supervised. The school needs to tell this boy's parents that if there is even one more transgression with your child or any other, this kid gets suspended. They are not three and curious and uninformed -- they are elementary aged and know that boys and girls are different, and touching private parts is not acceptable. Hiding means they know what they are doing should indeed be hidden. See the post above yours, about the mom whose eight-year-old son is touching girls' backsides at school. I'm surprised her son has not already been suspended and surprised that your school has not dealt harshly with this boy who has done this repeatedly.
You DO know that children can be, and have been, legally charged with sexual harassment for things as simple as kissing another kid? It can get silly, but it does happen. I bet this boy's parents don't know that. I am not saying this boy should have the cops called (yet) but he seems overly interested in body parts, which should lead to very serious consequences at school -- why hasn't it? It also leads to the inevitable question of whether he is seeing and hearing things at home that he should not be seeing or hearing, and whether someone else is showing him body parts inappropriately.
When you have your meeting, please try (I know it's hard) to be very calm and organized. Are you going alone or is your husband or SO going with you? I would be sure to take husband/SO. Have a script in advance and bring in notes -- plan what you are going to say and what you are going to demand of the school; do not wing it, because you will get emotional, sad, angry, and that will not help you or your child at all. Do not leave the meeting without a firm action list of what the school will do, and I do not mean some vague "We'll keep an eye on him" stuff.
This boy should be kept on the shortest leash possible for a time. I replied to the other mom in the other post about how our teacher in first grade handled a boy who touched girls' backsides (and acted out in other ways including very inappropriate sexual talk--he had older brothers who were very inappropriate and copied them). The teacher put his desk so it touched hers. He was never out of her sight. At group times he had to sit next to her. On the playground he was required to stay next to the teacher. (Did he hate it? Yes. So what?) When he got one toe out of line for any reason, he went directly to the principal's office and did his classwork sitting alone there with the principal or secretaries. We were lucky; this teacher had 25 years of experience and moved immediately, calmly and firmly to deal with this boy, and she turned him around by not yelling or fussing but just being clear that he had to be her shadow, period. He needed that firmness desperately and it helped him mature.
And critically, after there were some incidents where boys were having organized fights behind some shrubs on the playground, the school banned any kid from going in certain areas -- behind shrubs, in areas behind play equipment, etc. At recess, at least one teacher or aide was posted where she could see those areas at all times. A similar policy could help at your school -- the areas under slides etc. should be in the sight of an adult at all times, and if classrooms are arranged so there are areas where kids can hide and play doctor, there needs to be rearrangement right now.
One thing -- your daughter may be worried and upset that she "tattled and got Billy into trouble." That is very typical. Tell her as much as you need to that she did the right thing and should always tell an adult -- the first time anything happens. Be sure too that she understands that she CAN say no when another kid asks or tells her to do something; she does not have to "be nice" and agree to anything, ever. Girls are socialized from a very young age to be nice and try to get along with everyone, which can make them feel they shouldn't say no, and can make them feel guilty when they tell what's been happening. You need her to talk to you and not feel after this that she should clam up because she got another kid into trouble.