Is it legal for Waukesha, WI high school to place no left turn and roadblocks in their parking lot?

The owner of private property (here the city) can install traffic control devices on their property if they please.  The real question is, Can you be issued a ticket for violation of the sign while not on a public way?  Ordinarily, you can be issued parking tickets by police officers for violations on private property, ie. fire lane, handicapped spaces in shopping plaza lots, but not moving violations such as stop sign violations for signs in the parking lot.  Basically, the school can do it but they may have a tough time enforcing it.