Tahsiyn Ismaa’eel was determined to get her campers a full day in the pool.

Ms. Ismaa’eel, the director of Darul Amaanah Academy in Wilmington, Del., can see the closest city pool, Foster Brown, from the school’s back door. It’s where she has taken children attending the school’s summer Arabic literacy program for the past three years, usually three times a week.

But this summer was different. Since late June, the group was harassed on at least six occasions because of the children’s attire or barred from entering the pool entirely, Ms. Ismaa’eel said. Staff members had begun citing an unposted rule prohibiting cotton clothing in the pool, she said, seemingly targeting her campers, a majority of whom are observant Muslims who dress modestly and wore long shorts or tights, T-shirts and headscarves to go swimming.

On July 16, she tried one more time. The group arrived and was told the pool was at capacity, Ms. Ismaa’eel said. She and the children waited until a family left and they were allowed to enter, but the facility closed shortly after, and everyone was forced to leave.

She now takes her students to another pool, Eden Park, 10 minutes away, where their attire hasn’t been questioned.