What is the ruling on a teacher who combines the Asr, Maghrib, and Isha prayers and delays them until after the time for Isha due to giving lessons to students?

The mentioned teacher is sinful for delaying prayers past their prescribed times (Asr until Maghrib, and Maghrib until Isha), as this is not a permissible combination, and lessons are not an excuse for it. He must fear Allah and maintain prayer at its proper time, as he is a role model. He should be advised gently and wisely, and reminded of Allah's punishment.