There are a number of methods that are generally acceptable:
Hobbs time
Brakes off to Brakes on
Tacho plus a factor, usually 0.1
Airborne time plus 10 minutes or 0.2
If you are flying at a school they are generally interested in charging the most, hence the Hobbs. The Tacho is used for engine running time, but can read low if the rpm is low, and may be 0.1 low at the end of a PFL lesson. If its your aircraft the choice is yours.