Is it considered unusual in the United States for a professor to run a class outside on the grass?
A university courtyard is like a park and students are always hanging around.


There are thousands of colleges and universities in the United States. Some have large campuses with lots of space to hang around. Some are urban campuses with little outdoor space available. Some are entirely contained in one building. Some have mainly residential students, and some have mainly (or only, or some) commuter students who live off-campus. Some serve mostly full-time students, while others cater to students who are doing other things outside of school and don't have much free time to hang around. Some are in boring locations with not much else to do, and some are in fun cities with many events and activities available outside the university. Some are in pleasant climates and some are in places that are very hot or very cold for some or part of the year.

All of the factors above and many more go into determining whether a given university, out of the thousands, is one where students tend to hang around outside.


  He also mentioned that it is not unusual for a professor to gather students and sit on the grass outside and run a class.


It may happen occasionally (see example), but it is very unusual. I have never attended, taught, or seen such a class firsthand.

Here is an article with some more anecdotes.


  He mentioned that the university environment is not limited to formal education (what is the case in Europe more or less). 


If a professor decides to teach outside, it's still a formal class, not a social event.