A private seminar for councillors in Stornoway on Tuesday approved plans to prepare reports on as many as 22 closures.
Neil Galbraith, the council's director of education, said: "The review is necessary because of the declining resources in terms of revenue and capital. The Audit Commission report on the number of empty places in schools has added to the pressure. There are too many schools and it is a question of rationalising them down to a reasonable level."
The plans will entail more travel for pupils, but the council already buses all pupils who live more than one mile from school.
Jeff Sedgley, local secretary of the Educational Institute of Scotland, said the union was not expecting closures on the scale proposed. The union supports the principle if education can be improved and there are no compulsory job losses.