Replacing 5 panels in a High Schoo (120/208v panels). All branch circuits feed lighting and receptacle loads. They are very old and also "main lug only". Do I have to have a main breaker in the new replacement panels? If so where is this in the code?

Mike

PER NEC, MLO panelboard is fine. A MCB in each panelboard is not required, but optional, as the panelboard will be protected by the feeder CB at the distribution board.

You should be checking with the engineer that wrote the project specifications, and/or the LAHJ.