Is it considered a lie for a university-educated engineer to claim his degree is a high school diploma to avoid ill-treatment in temporary jobs below his qualifications?

Lying is forbidden, and it leads to immorality and Hellfire. If you are bothered by mentioning your university qualification, then use ma'areed (allusions), which is equivocation with a word that bears two meanings. For example, if someone asks about your qualification, you can say, "I have a high school diploma," so your words are true, and you are saved from lying.