So what were they teaching?
This might give us an inkling
what they were teaching
Terrorism Act 2006 | Home Office
# Encouragement to Terrorism
This makes it a criminal offence to directly or indirectly incite or encourage others to commit acts of terrorism. This will include the glorification of terrorism, where this may be understood as encouraging the emulation of terrorism.
I suppose an 'I love Osama' on a tshirt could be 'glorification'.
if you were twisted enough to wear one