The practice has become so widespread that the emirates' education chief, Mohammed Ali Abu Lailah, has warned the errant teachers they will face the sack unless their attendance improves.
"These people are not fit to be in the teaching profession, which is a sacred profession," he said.
He proposed he introduction of "academic centres" where teachers could make up the classes they had missed.
Oil-rich Abu Dhabi is by far the wealthiest emirate in the UAE, and Ras Al Khaimah, in the relatively poorer northern area, frequently complains that it receives inadequate federal funding.
Teachers say they bear the brunt of the funding shortfall and that is why increasing numbers are seeking to supplement their incomes by working part time elsewhere. Under UAE law, teachers cannot hold other jobs.