What qualifications are required to become chief instructor at a 141 flight school (FAA)?
The requirements are in 14 CFR 141.35, Chief instructor qualifications. They're too long to quote here in full and the exact requirements depend on the type of instruction, but in summary:


Hold a commercial or ATP certificate
Hold a CFI certificate
Pass knowledge and proficiency tests
Meet minimum flight time requirements (depending on the type of instruction, e.g. 1000 hrs PIC time is required for private pilot instruction)
Meet minimum flight training time requirements


For training pilots for an instrument rating, the chief instructor also needs an instrument rating, instrument flight time and instrument training experience.