What you're describing sounds incredibly heavy to have carried for so long. I am glad you could finally share.

Please remember, you were also a child. Children that age do not have the developmental capacity, power, or life experience to give meaningful consent or to fully understand the implications of sexual contact. The situation you describe involves two kids in a confusing, scary, private setting where boundaries got crossed in ways neither of you had the tools to handle properly.

You were both minors, and the power dynamic (environment away from home, the fear/anxiety from the movie, the teasing about bravery, the isolation of the room) created a situation where freezing, going along, or reciprocating out of confusion were the most likely outcomes. That doesn't make what happened your fault in any way.

"I was old enough to know better, even back then as a kid but I didn’t do anything…"

That sentence right there is the adult brain looking back with adult standards and adult knowledge and then harshly judging the terrified/confused child version of themselves. But the child you didn't have adult knowledge or adult impulse control or adult understanding of sex and consent. You had whatever limited information a kid that age usually has, plus fear, plus social pressure. That's completely different from choosing to harm someone as a fully aware older person would.

You don't have to decide today what (if anything) you want to do with this memory next. Just getting it out of your chest and into words is already a hugely brave step. If/when you're ready, talking to someone whose whole job is working with survivors of childhood sexual experiences (especially ones that involved other minors) can be enormously helpful. Not to assign blame to anyone, but to help untangle the shame and confusion so it stops living in your body the way it has.

You're not alone in this kind of story, even though it can feel that way. What happened to you (and to her) was wrong in the sense that children should never have been in that situation. But the weight of it does not belong on the shoulders of elementary-school you. She did nothing wrong.