Are the Dhuhr and Asr prayers considered valid if they are combined or delayed beyond their prescribed times due to work circumstances in teaching?

One must be diligent in performing prayers on time. It is not permissible to combine two prayers except for a legitimate Shar'i excuse, such as travel. What you mentioned in the question is not an excuse for combining prayers as long as you are able to perform them on time. The solution is to pray Dhuhr at its designated time, and to pray Asr before Maghrib, even if the preferred time for Asr has passed, based on the Prophet's (peace be upon him) saying: "Whoever catches a single Rak'ah of Asr before the sun sets has caught Asr." As for past prayers, you are not obligated to make them up because they occurred due to a permissible interpretation. However, if you make up what you performed before its time, it would be better and more absolving for your conscience.