It depends on the weather. Sometimes our winter weather is in the 60-70 degree range. Now if it's less than 60 degrees out then I would encourage them to wear pants but typically what they wear is a battle I choose not to fight.
The boys, no way. Dd- only if she wore really warm tights under them. Luckily, ds1 has no desire to be that cold (he's not into looking cool), and I still have ds2 and dd convinced that they can't wear them at all in cold weather, partly b/c it's too cold and partly b/c Mom said NO.
I would, too. I work at a major university in the Midwest and I recently saw a guy dressed in shorts and flip flops on a cold snowy day and that is clearly inappropriate and even dangerous- he could easilty fall in flipflops and he should have learned from his parents that dressing appropriately for weather conditions is part of being a sensible adult. Mankind has advanced because we have learned to adapt to varying climates. This kid didn't get the correct instruction from his undoubtedly lazy parents.
My kids only wear pants/jeans if I force the issue (not too often - we live in Florida). I don't care if they wear shorts and t-shirts. I do have them put a sweatshirt/jacket in their backpacks if they don't want to wear it but I drop them off at school right at the door and they go directly inside. My oldest dd picks them up at the door so really they aren't outside while at school.