Our kids (dual nationals) are US Citizens and all have been blocked from attending local schools.
The principal told us have the consulate or embassy write a letter to the education bureau requesting assistance and as long as space (as per JanJal noted above) is available - they can be admitted.
Only problem with this process - the US Consulate American Citizen Services refused to write the letter. They'll fly out to ensure a convicted drug dealer and terrorist is treated well - but writing a letter requesting equilateral and equivalent support for the children of US citizens is apparently impossible.
Not particularly pleased or impressed with US Citizen Services...or my perceived lack of.