The pen is similar to an e-cigarette and can be bought for $25 in grocery stores.
It does not emit an odor when used with marijuana and is easily used in school, KCNC-TV, Denver, reported Thursday.
"Teachers are aware of what to look for. The nervous habit of biting on your pen has a new meaning to it," said Ron Castagna, principal of Lakewood High School in Lakewood, Colo.
"You can use them for weed (or) hash oil. Pens that come with capsules with weed in them melt the plastic and cook the weed," Tim Martinez, the school's security director, said.
"I've seen people in my math class,' said student Jack Maestas, "and they'll be sitting there with the teacher and they'll be smoking it, and they (teachers) won't even know it."