A problem with the heating system caused classes at Hempstead Middle School to be canceled Tuesday.Students were scheduled to begin their statewide English and Language Arts testing, but school officials say the heating problem would have made it difficult for them to concentrate on the tests. The school?s 1,400 students were told not to come in, although some didn?t receive the message and came in anyway. Those students were eventually picked up by their parents.These types of situations in the Hempstead schools have been getting a lot of attention because of serious academic and infrastructure problems in the district. District officials say they will replace the entire heating system this summer. In the meantime, the problem will be fixed in time for students to take the exams Wednesday.