If you are providing the airplane to transport the coaches:
You are a commercial operator and require an operating certificate under part 119 to fly under part 135.
If the coaches have an airplane (rental or owned) and are simply hiring you to fly it for them:
You may provide your pilot services and act as PIC of their airplane and be paid while transporting the coaches as allowed under 61.133.
Your commercial pilot privileges are laid out in 14 CFR 61.133.
§61.133 Commercial pilot privileges and limitations.
(1) General. A person who holds a commercial pilot certificate may act as pilot in command of an aircraft—
(i) Carrying persons or property for compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation; and
(ii) For compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation.
This says that a commercial pilot you may be paid to act as pilot of command.
To constrain this, you must also look at 14 CFR 119.1. The important part to look at herei s 119.1(e):
(e) Except for operations when common carriage is not involved conducted with airplanes having a passenger-seat configuration of 20 seats or more, excluding any required crewmember seat, or a payload capacity of 6,000 pounds or more, this part does not apply to—
(1) Student instruction;
(2) Nonstop Commercial Air Tours conducted after September 11, 2007, in an airplane or helicopter having a standard airworthiness certificate and passenger-seat configuration of 30 seats or fewer and a maximum payload capacity of 7,500 pounds or less that begin and end at the same airport, and are conducted within a 25-statute mile radius of that airport, in compliance with the Letter of Authorization issued under §91.147 of this chapter. For nonstop Commercial Air Tours conducted in accordance with part 136, subpart B of this chapter, National Parks Air Tour Management, the requirements of part 119 of this chapter apply unless excepted in §136.37(g)(2). For Nonstop Commercial Air Tours conducted in the vicinity of the Grand Canyon National Park, Arizona, the requirements of SFAR 50-2, part 93, subpart U, and part 119 of this chapter, as applicable, apply.
(3) Ferry or training flights;
(4) Aerial work operations, including—
(i) Crop dusting, seeding, spraying, and bird chasing;
(ii) Banner towing;
(iii) Aerial photography or survey;
(iv) Fire fighting;
(v) Helicopter operations in construction or repair work (but it does apply to transportation to and from the site of operations); and
(vi) Powerline or pipeline patrol;
(5) Sightseeing flights conducted in hot air balloons;
(6) Nonstop flights conducted within a 25-statute-mile radius of the airport of takeoff carrying persons or objects for the purpose of conducting intentional parachute operations.
(7) Helicopter flights conducted within a 25 statute mile radius of the airport of takeoff if— [snip]
(8) Operations conducted under part 133 of this chapter or 375 of this title;
(9) Emergency mail service conducted under 49 U.S.C. 41906; or
(10) Operations conducted under the provisions of §91.321 of this chapter.
When you put those together you can always be paid to be PIC but if you provide the airplane and become an operator, then there is a very small list of things you can do without an operating certificate.