Is giving an answer in an exam considered cheating, even though giving answers is not allowed, and does the hadith "Whoever is asked about knowledge and conceals it will be bridled on the Day of Resurrection with a bridle of fire" apply to it?

Your answer to the examinee during their exam is forbidden cheating. This has no relation to the hadith: "Whoever is asked about knowledge and conceals it will be bridled with a bridle of fire." This hadith is not general; rather, it is specific to knowledge that is obligatory to teach, such as teaching Islam to a disbeliever, or issuing a fatwa regarding what is permissible or forbidden. As for knowledge that is not necessary to know, or if the answer would lead to a greater harm, it does not fall under this warning.