Is attending lessons that prevent one from performing the prayer at its appointed time considered an excuse for delaying the prayer, or is that a sin, especially if the time of the lesson cannot be changed or the lesson cannot be abandoned?

It is regrettable that many Muslims are negligent in the matter of prayer. One manifestation of this negligence is allowing the prayer time to expire without concern, especially when attending classes. Delaying Maghrib prayer beyond its appointed time without a valid excuse is impermissible; in fact, it is among the major sins. Attending classes is not a valid excuse permitting the combining of prayers. The duty is to advise the teacher and students gently and kindly to maintain prayer at its appointed time. If they do not heed the advice and attending the class is necessary, one can attend it while in a state of ablution and perform the prayer when its time begins.