Credits, while not required for elementary and middle schools, are still used to calculate GPAs by weighted average. For example, if you want the academic classes to weigh more than PE in the GPA, you would give them more credit. Make the credits a multiple of the type of marking period. Trimester credits should be a multiple of 3, and quarters a multiple of 4.
The reason for this is that each trimester divides the total credits by 3 (and quarters by 4) so each trimester or quarter receives one-third or one-fourth the total credits for the year. If, for example, you want the academic classes to weigh twice as much as PE in the GPA, under a trimester system you would make the academic classes worth 6 credits and the PE classes 3 credits, or in a quarter system you might use 8 credits for academic and 4 credits for PE. This becomes very important because parents and students often divide reported GPAs by the number of classes without understanding the underlying weighted calculations and then insist that the calculations are wrong. They're not.
If you use some trimester- or quarter-length classes (as opposed to yearlong classes) such as electives, then those courses should only receive 1 credit since they only exist for one marking period.
In summary, yearlong classes should receive the total credits to be divided by the number of marking periods, and quarter or trimester classes should receive one-third or one-fourth the yearlong credits. High schools generally have 10 credits per full year course and 5 per semester as specified by Ed Code and college entrance requirements.