How can I cycle in winter while not being able to change work clothes? I started recently to bike to work, where we have a shower and I can change clothes. However recently I also started attending an evening course, where there is no place to change.

So when I commute to work, it's easy. At home I wear my warm cycling clothes and then at work I switch to light comfort ones.

However the problem happens when I want to go to the evening course. Since it's pretty warm at the classroom then I can't wear my warm cycling clothes and go to the course and stay with them for 2 hours. And since there is no place to change, then I can't change there to be able to return home from the course.

Any ideas on how to solve this? or any clothing alternatives/ideas?

My commute is pretty long. From the course to home is about 18KM.

Edit: the temperature in winter here gets easily below 0, and the worst is -18 to -20.
I've had success wearing legwarmers under trousers during winter.  

The main advantage is at the far end you can either wiggle them down while standing, or drop your pants in the toilet cubicle and shove them down around your ankles for comfort.  

This works well with overshoes, in combination with a decent jacket and gloves.  I also wear a neck buff and/or helmet liner.

Try sitting nearer the window or door if you need a cooler breeze.