English equivalent to the Indian saying "Like a thief being stung by a scorpion"

Solution 1:

I think you are referring to "திருடனுக்கு தேள் கொட்டின மாதிரி"(this is the original Tamil saying that I am aware of. It could exist in other Indian languages too)

You can say that the thief found himself between a rock and a hard place. He could either shout which will eventually lead him to his capture or bear the pain which could probably kill him. The idiom has the same implications as "between the devil and the deep blue sea"

(prov.) if you are between a rock and a hard place, you have to make a difficult decision between two things that are equally unpleasant

[The Free Dictionary]

As far as the second scenario is concerned, the student has 2 difficult choices to make :-

• He can either disclose that he skipped school/field trip and avoid vaccination. However, this could potentially infuriate his parents for lying to them.
• He can go ahead with the vaccination and conceal that he skipped school/field trip. But vaccination gives him all the jitters, so he's really has no other way out.

Update

The English idiom tends to be associated mostly with misdeeds, i.e. people putting themselves in such precarious situations. However, the original South Indian(Tamil) saying can also apply to situations where you try to do good but inadvertently end up like "a thief stung by a scorpion", where the choices in front of you are limited and only detrimental.

Solution 2:

Your examples seem to be leaning towards unfortunate things happening to people who put themselves in that situation in the first place, in which case one could be said to be

hoisted by (some people say "with" or "on" instead) his own petard

which basically translates to

to be hurt or destroyed by one's own plot or device

A petard is, or rather was, as they have long since fallen out of use, a small engine of war used to blow breaches in gates or walls. They were originally metallic and bell-shaped but later cubical wooden boxes. Whatever the shape, the significant feature was that they were full of gunpowder - basically what we would now call a bomb. (Phrases.org.uk)