Lent ends when the evening Mass of the Lord’s Supper on Holy Thursday begins because the Mass of the Lord’s Supper ushers in the Holy Triduum, a liturgical season in its own right and the shortest of the liturgical year. For more information, click here.
As for the Lenten penances, those are voluntary practices that people take up as personal devotions in addition to the penances required by the Church on the Fridays of Lent. Voluntary penances can be voluntarily set aside, so if your students wish to take a break from them on Sundays in honor of the Lord’s Day, that is perfectly fine. That said, the Sundays of Lent are indeed a part of the Lenten season. See the links below for more information.
The Duration of Lent** by Jimmy Akin
The Length of Lent by Jimmy Akin
Forty Days by Jimmy Akin