The POH states "For a short field, 10° wing flaps and an obstacle clearance speed of 56 KIAS should be used." Take-off distances are always provided for a nominal obstacle height of 50 ft and they have determined that 56 is the optimum nominally. In that text on page 4-32 of "If an obstruction dictates the use of a steep climb angle, the best angle of climb speed should be used with flaps UP and MCP. This speed is 62 KIAS at sea level, increasing to 67 KIAS at 10,000 feet." the scenario is beyond the typical obstacle near the end of the runway and the pilot would normally accelerate to the speeds as appropriate mentioned on that page 4-32.
A large flight school here using that type leans on the ground per the POH. What you have been told by other instructors - I suggest that you seek a reference to the SOPs or is it just individual opinions? Maybe there have been incidents of students attempting take-off with the mixture leaned and they have decided to accept the risk of plug fouling instead (better to lean more aggresively so the student gets an obvious message from the engine when increasing power).