Hello,

It is a bit difficult to say as you have not mentioned your board for 12th standard. I assume if the subject was not in your best of 5 or 4 based on your board then no I do not think it would affect you because generally it doesn't . Moreover even if you failed  they would conduct a supplementary examination within a month which you can give and improve yourself. And as for the teaching exam it also depends on which subject you are giving your test if it has no connection to the subject you failed in then I do not think it would affect you.

Hope it helps.

Good luck.