I have to agree with Sally - how do you both know it's only two hours? It's possible she is getting more sleep but 'feels' awake and 'ticks off the hours' before dozing again.
Does she catch up with sleep at any point?
Anyway, regardless of that, this is something you have to push for more investigation because it can have repercussions. If you need to get the school nurse involved, do it. Keep pushing. And then push some more.
Try to break the cycle somehow - there are loads of options; try a completely different bedtime routine, be drastic - perhaps even let her fall asleep on the sofa by you one night. Lavender and clary sage essential oils are good, just a drop or two on her pillow. Just brain dumping now, so probably not making much sense, sorry.
And yes, I've been there. Still there actually and DD is now a teenager. She was a poor sleeper from birth. There was a short period during primary school of sleeping well, but she went back to 'her normal' from puberty, at about 10 years old. Now, if she could sleep her own hours, she would probably sleep from 4am until noon. Typical teen, I guess. Unfortunately she has to get up at 0530 for school. Last night I should think she had about 4 hours of sleep max.
I hesitated about sharing the next bit, but feel I should - DD now has mental health problems, but rather than these being caused by the lack of sleep, her doctors feel her sleep pattern is a symptom instead. She is currently having investigations for epilepsy.
Sorry for probably worrying you - hopefully unnecessarily - but if at all possible, I would be pushing for an EEG exam.
FWIW, I too have little need for sleep and average 4-5 hours a night with occasional nights of none at all. Ive always been like this, by the way. Apparently my brain is fine and I don't have MH problems.
Again, I apologise for the length of this post, and the content. I truly hope you and your DD can find your way through what may be just a blip.
Happy for you to pm me if you wish. Good luck.