Why do guys hang around the hallways in groups? And why do guys laugh and stare when girls walk past?

i go to a technical school in the morning and school also. there are these guys at tech school that always laugh at my friend angela and since then she tries to avoid going that way. i have to p those guys cause my cl is toward the front of the school, when im with my friend scott or my gay friend jay. they stare but when im walking all by myself they give me looks that they don't give me when im with scott or jay when i walk past them .one time i was wearing this shirt/dress wit black leggins & black heels because it was picture at school and i was hoping to get a new id for school. i sat down at a wooden table and the guys were standing around me, the tall guy sat on the edge of the table and then the other 2 were both the opposite side of the door way which has 2 doors. they were talking and i was just sitting there quietly this what i thought in my mind( why the hell are they standing here instead of going to the others spots in the hallway) i walked away and they were staring at me( checking me out). why do guys hang around the hallways? like they are always on both sides of the hallway at tech school and u gotta walk throught them, also the crowd of other students. whats up when guys laugh and stare when girls walk past alone or with friends. im doing Early childhood Care at tech school and next week on thursday i turn 17.