5 year old exposing genitals at school

I never thought I’d be writing this. At least not until much later on. My son just started kindergarten and already I have been called by the vice principal and been told he allegedly showed a girl his penis. I was so shocked. I say allegedly bc that’s the word he used. He said no one witnessed it. It’s her word against my son’s and at the time of the call neither child gave any info besides the girl going home and telling her mom what she saw I assume. That’s what prompted everyone. I picked my son up from school and calmly asked him what happened. He told me another child told him to do that and he said he didn’t want to. My son doesn’t typically lie but I know that lying is common amongst kids so I can’t rule that out for sure. I’m hoping he’s being honest. I had to go into detail on why we don’t do that and when someone tells you to do something like that you say no and you tell an adult. I even asked if he had been touched and he said no never. I’m in shock. I want to cry. I know some of this behavior is “normal” but I feel so bad for both kids involved. I’m going to the school tomorrow to give them the story and hopefully the 3rd kid tells the truth If that is the truth. We try so hard to protect our kids from bad things while still being open and informative. I’m not trying to shelter him completely, but some things aren’t appropriate for 5 year olds. I want to believe in my heart he didn’t do it on his own, that’s not like my son. Idk what to do here, I feel lost and I feel like a failure as a parent. Has anyone else dealt with this?