Boys keep asking about my underwear, like saying send me some pics and other rude things, please help! ?

Some boys I know are asking/ hinting what color my underwear is :/ Like at school there always trying to look up my skirt so they can see my knickers, or on a hot day if i take my blazer off, they say oh nice bra, and slapping my bum at school, ( I'm defiantly not a slut, even thought it might sound like it) anyway there really addicted to looking at me, or when i was in town and had l black leggings on, they were walking behind me to see threw them, and it's abit creepy, can anyone please help, tips/advice to get them to stop? Thankss..