All of your sentences are grammatically correct, but sound awkward. I put sentences with the same meanings next to them for reference. 1、We all felt that to be the highest praise. --> We all felt that to be of the highest praise. 2、They heard him to be very foolish. --> They heard he was very foolish. 3、The boy soon perceived the snake to be a dead one. --> The boy soon perceived the snake to be dead. Also, for your question title, make sure you ask "Are these sentences right?" because "sentences" is a plural word, you need to use "are" instead of "is." I hope this helps :)

Grammatically, they are correct, but the style is a little odd.

We all felt it was the highest praise given by the teacher this whole year. We all felt (that) he deserved the highest praise . All of us felt that his efforts should be accorded/given the highest praise. They heard that he is a real fool/very foolish person. The boy soon realized the snake was a dead snake (but not before thinking it to be a live one).