Is there anything in NFPA72 that would require an installer or a design professional to install or design an installation of a fire alarm system to meet the requirements of a Class A circuit?

Secondly- what percent of the systems being installed are Class A?

I see class 1, 2 & 3 circuits in the NEC, but no class A circuits. What are they?

A class A FA system uses return conductors for supervision. A class B uses EOLs for the supervision of the circuits.

I have seen more Class B lately. Last Class A I put in was at the Maine State Prison.

George
Your answers on Page 72-41 Chaper 3 (3-4)
1999 NFPA 72.
Most nowadays are Class B Class A requires the outgoing and return conductors to be routed separately and shall Not be run in the same cable assembly. Page 72-41 (3-4.2.2.2*(Note exceptions) on page 74-42

Depends on the type of buildings you work in. More than 50% of the building I work in are Class A due to reliability requirements and occasional slow repair cycles.
Many are not familiar with Class A circuits and they are not specified due to "that's the way I always did it, and it worked that time"

Most of our systems use class B, where you see 2 jumpers in the panel per zone and 2 wires going out. Class A is safer because it can be cut in the middle of the loop and still cause an alarm on either end. A class B intelligent system may not use EOLs because each device is polled so you don't need a resistor, capacitor, or diode at the end to know that you're getting to the last device.