All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There are four situations where a Muslim could perform Tayamum instead of ablution:
1- When a person does not find water in the time specific to that prayer and does not expect to have it in that time (during a trip for instance).
2- When there is water but the person fears for his life or might physically be harmed, by a bad sickness or a great pain (wounds…).
3- When the water is near and available. But one fears for his life, family, wealth etc.… because of a snake, a war…
4- When a person fears that the time of the prayer will be over before he gets water. But if a person does Tayamum only because he is in a hurry for a lecture or he finds it difficult to take off his clothes or other such reasons, then this is not a sound reason as it is only a kind of negligence.
In such cases, one should make up for the prayer (i.e. do it again with proper ablution).
Allah knows best