While in 5th we had to send in our paperwork to ECFMG and the school has to verify that we are indeed students. I filled out my paperwork about halfway through the semester and the school took about 4 weeks to verify everything. I think overall the registration took about 8 weeks so I guess it's possible to take the Step while in 5th.
As far as will the school let you, I have no idea. Personally if you play your cards right you can utilize 5th semester to study for your step and take it right after 5th.