SOMERVILLE, Mass. — A Massachusetts kindergarten class is drawing attention for using a lullaby to teach students about lockdown drills.

Taped to the classroom's chalkboard is a rhyme set to "Twinkle, Twinkle, Little Star," rewritten telling students what to do if there's a shooter.

It begins "Lockdown, lockdown, lock the door. Shut the lights off, say no more."

Parent Georgy Cohen noticed the poster while visiting the Somerville school Wednesday. She took a photo and posted it on Twitter, writing, "This should not be hanging in my soon-to-be-kindergartener's classroom."

Her post had been shared more than 18,000 times by Thursday afternoon and drew hundreds of comments. Many said it's disturbing that children so young have to be prepared for a shooting.

Cohen told The Boston Globe the poster was "jarring," adding that students didn't have to deal with those types of threats when she was in kindergarten.

She and Somerville Public Schools declined to identify the school, but district officials confirmed it was the work of one teacher and is not used across all of the city's schools.

The Somerville district enrolls about 5,000 students in a suburb north of Boston.

In a statement, Superintendent Mary Skipper and city Mayor Joseph Curtatone applauded the teacher's creativity, but they lamented that lockdown drills have become as common as fire drills.

"As much as we would prefer that school lockdowns not be a part of the educational experience, unfortunately this is the world we live in," they wrote. "It is jarring — it's jarring for students, for educators and for families."

The poem, which is written in bright colors on oversized lined paper, ends on a lighter note than it begins: "Lockdown, lockdown, it's all done. Now it's time to have some fun!"