There are positive and negative aspects to open the school gates during the break-times, so people have different opinions on this subject.
On the one hand, there are some advantages because students sometimes need to do photocopies and in the street they are cheaper. Also, students need to feel wide-awake of school and exams routine at least twenty minutes. In fact, students that are allowed to leave are old enough to know what to do.
On the other hand, there are many teachers who think that letting students go out in break-times would be a lack of control because it will be impossible to control which students have left and who have not. Also, in the high schools there is a photocopier available for students and a canteen to have a snack or a drink.
In conclusion, I believe that students have all they need inside the school and it is not necessary to go out in break-time and this way is better also for teachers.