Over 200 schools across Ireland are each spending upwards of a €100,000 a year in rental costs for prefab accommodation.
Department of Education data released to Fine Gael shows that a total of almost 400 schools are paying upwards of €50,000 a year to rent their prefabs.
The information also shows 180 schools paying private companies between €50,000 and a €100,000 a year in rent.
It is expected that up to €48m will be spent on school prefab rental this year.
The Department says a review of rental policy is under way and value for money is a central aim.
A new policy favours the purchase of prefabs in schools where they will be used over a long time period.
Fine Gael said it is an insult to children, parents and teachers who are seeking new school buildings.
Speaking on RTÉ's Morning Ireland, Fine Gael education spokesman Brian Hayes said it was a scandalous waste of money.
In response, the Minister for Education's spokesman said it is now the case that if temporary accommodation is required for more than three years the Department will offer the school a grant to buy it rather than to rent it.