If girls become wary of you when you tell them that there is a spider in their hair, you can convince them that no, really - there really is a spider in their hair this time - by saying "no, really - there really is a spider in your hair this time".
If more persuasion is required;
Level 1 : There is a spider in your hair.
Level 2 : No really - there really is a spider in your hair this time.
Level 3 : Oh God, there's a really big spider in your hair. Everyone, come and look at the really big spider!
Level 4 : It... it looks like it's laying eggs...
Level 5 : Look, I know I've been saying this a lot recently, and at the back of my mind, I realised a time would come when one day, you really might have a spider in your hair. I think I was hiding from that possibility, hoping it would never come, because I knew you wouldn't believe me when I told you. But honestly, this time, there is a massive spider in your hair, and from the markings I think it's poisonous. I don't expect you to believe me, I guess I've dug my own grave in that respect, but please - please seek help regarding the oversized spider that's running amok in your lovely hair. You must tell me your hairdresser, by the way.

But there was a spider in my hair. Naturally I was sceptical, so an entire changing room full of girls was unable to convince me of my infestation until I looked in the mirror, at which point I screamed and thrashed for an indeterminate amount of time.
Then the swimming teacher came in and sprayed the offending spider with window cleaner until it died. Yes, the swimming teacher. We were just that posh.