Oh, I wouldn't trust those witnesses tbh. No proof that they are telling the truth. They could have planted the pens on her. Could be a massive wind up and agree that the bullying theory is very plausible.
I too find it hard to believe the set of gell pens cost £15.00 - must be at the luxury end of the market. I can buy a set for £1.00 at our Pound Shop.
I think you have to believe your daughter for now. Say to her this so called 'theft' must be a one off. By the law of averages these things hardly ever happen. So if someone accuses her of stealing again in the next few months, you will have to look into it much more closely as this is so unlikely a coincidence. It means someone, very possibly the witnesses, are not telling the truth.
If by any remote chance your dd did steal the pens could this have been in reaction to you being called in to see her teachers about her lying? perhaps she was upset?
Did other girls know you and your dd had to see the teacher? perhaps your dd was being teased about it and someone took it further by setting her up as a thief? horrible thought
No way would I trust the witnesses against my child.
I would sympathise for the 'loss' not 'theft' of the pens to the girl's mother - careful not to assume responsiblity, then see how things develop in the next few days.
Sadly, if you offer to replace the pens, it will look like you believe your dd stole them, adding more to the story that your dd is 'the thief'.