It used to not be possible because of their not being a governing body. However times have changed and the Freestyle Football Federation is the World governing body for freestyle football now and you can point that out to your P.E. staff.
Im certain you can be assessed on Freestyle now, its just about finding the right way to do it. I tried very hard to be assessed and the only prevention was that before it had no governing body. Now it does im certain it is possible. You just have to get someone to judge you on it....... and match you with a rank, this will be tricky due to freestyle not having set points for certain skills and also the fact that you havent competed in many credible competitions e.g. Redbull, prague, wffc etc. I still wouldnt imagine this would prevent you as in football if you play for a regional club you can still get like a c grade.... I think the best way to do it would be to have the P.E lecturer there with you and film you doing a performance to the camera of 2-3 minutes or something and to send it to the federation for judgement. --- but this is just what I would imagine would be the best way of doing it and not what is the way

I did a similar thing with martial arts and it worked fine....because the p.e. staff not knowing anything about it doesnt make a difference.
Try speaking to the federation about it. ( F3 ) - freestyle football federation.
There website is -
http://freestylefootball.org/ - There is a contact number and email on the contacts page.
Goodluck mate
Andrew