Yes, the school counselor may seach your child's backpack at school. The school--and hence its staff or employee, like the guidance counselor--are "in loco parentis": that means, that the law makes them effectively the parents' "stand in" while the children are in school, with much of the power that parents would have over their children (during school hours and on/at the school's location). This is necessary to allow the school to exercise authority or discipline over students. This power extends to the ability to authorize searches of the student's property or belongings on the school grounds, so the school, through the guidance counselor, could authorize or allow the search, the same way that you could have.