Children taught by staff who aren't their usual teachers make less progress in lessons, according to a joint report by the school's inspectorate Estyn and the Wales Audit office.
The report says that supply teaching is often less effective because staff don't know enough about the needs of the pupils they teach.
It added that the pace of lessons can also be slow and expectations too low.
Authors have recommend that supply teachers gain better access to the national training programmes that are available to permanently-employed teachers
The Welsh Government says it will consider the report's recommendations and respond in due course.
More top news
The singer said he often dreams he is with his late wife - only to wake up and remember his loss again.
Sunny spells will develop but the winds will be strong, especially along the coast and over high ground. The odd light shower is likely too.
Theresa May promises a new relationship with governments of Wales, Scotland and Northern Ireland ahead of Brexit talks between the nations