más = more, bajos = short. más + bajos = shorter. There is no "more shorter" in English, although there is 'much shorter', but that doesn't apply here as there is no 'mucho'
Does that mean "mucho más bajos" is the correct way to say "much shorter"?
There is the English phrase, "much shorter" which is what I assumed the translation of "mas baja" would be. Nope. The nuance in English is, "shorter" can be any amount so long as it's shorter, while "much shorter" is relatively a lot shorter. Is there no such nuance in Espanol?
If they were standing a risers on a stage, could one not say "lower than" the same way?
In the given sentence ""Ellos son más bajos que el maestro" they use the masculine form "bajos" (short), thus requiring "ellos". To use "ellas", you would have to change "bajos" to "bajas".
"ELLAS son más BAJAS que el maestro."
In correct (US) English, the comparative is shorter. The superlative for comparison of 3 or more is shortest. There is no "more shorter" that I'm aware of.