(Reuters) - Idaho became on Thursday the first U.S. state to require two online courses for high school graduation when state officials approved the requirement.

The requirement, which goes into effect next year, was part of an overhaul of public education pushed by Tom Luna, the state's schools chief, and approved this year by the Republican-led legislature and Governor C.L. Butch Otter.

Luna said the measure, which mandates two of 46 courses required for high school graduation be taken online, would ensure students will gain the critical digital learning skills they need to be successful in the 21st century.

The rule makes Idaho only the fourth state to mandate any online instruction for high school students and the first to require two online courses for graduation.

But it has come under criticism from the Idaho teachers' union, which called the move a state-based mandate lessening local control of schools.

The decision to take online classes should be made by students and their parents, not by the state, Idaho Education Association officials said in a statement.

In addition to paving the way for online credits, the education overhaul, spelled out in three separate laws, curtailed the collective bargaining rights of public school teachers.

A parent-led group seeking to repeal the laws, including the online component, in June collected enough signatures to place the statutes before voters on the November 2012 ballot.

Alabama, Florida and Michigan are the only other states that require an online course for graduation, said Susan Patrick, president of the non-profit International Association for K-12 Online Learning.

(Editing by Dan Whitcomb and Cynthia Johnston)