Folded bikes are not bike by definition
They in most cases are no longer have "two wheels inline.
I took my folder in to lessons quite frequenty, I mainly locked it up outside in a safe area. But if lesson on other side of campus, train running late or I need to get away quick I took bike into class room.
Never a problem, however if lots of people started doing it......................

Often some people object to the fact its called a bike.
I have had problems on city buses in nottingham, despite the fact that it was smaller than other items on buss; ie large bags, ,but it never stopped me taking it on anyway!!

If its in a bag you ar proberlbly OK
Else sometimes you moght suffer from the :"quick this is unusaull, lets think of a general rule to exclude this. "Ok its still a bike", or "health and saftey", actually "we are not insured"............................." i dont like you"............"actually i am sad".............."borred".....................