Normally, the practice in my experience is to fill the tanks in all cases to minimize the situations in which fuel starvation would be possible. For this reason, in the United States most trainers are rented wet. Exact fuel calculations usually are only relevant when flying cross country, and even then using generous reserves is a wise practice. During a lesson there are usually lots of unplanned activities, like the ole 30 minute run-up, waiting on the taxiway to take off behind 3 other aircraft that have some kind of problem, and in-flight distractions, so cutting it close on fuel is a bad idea.
A second issue is that if you fill the tanks only for what you need, any additional rentals will each have to fill the tanks, too, which can cause delays and throw off the rental schedule. For example, let's say the C152 is rented 4 times that day. If each pilot only filled the tanks for what he/she needed, then the fuel truck would have to come 4 times. Sometimes it takes a while for the fuel truck to arrive. If you need to taxi to a pump, that is another delay.
For all of these reasons, the normal practice for a student is to fill the tanks. If you are the owner/operator of the aircraft, you may choose to parsimoniously load the fuel you absolutely require, and save some money, but it does not sound from your post as though this is the case.
IMPORTANT CAVEAT: Overweight
If the aircraft would be overweight if you were to fill the tanks, then the above advice obviously cannot be followed. In that case, assuming it is a rental, the logical thing to do is two computations: your estimated need (which are already doing), and the maximum possible. If the need is greater than the maximum possible, you will need to change your flight plan. If the maximum is significantly greater than the need (enough for two or more lessons), then you should fill to the maximum, so that the next student does not have fill the tank, too. If the maximum is only moderately over the need, and the next student will have to fill it anyway, then you may as well fill it just to your calculated need.
As far as your calculations themselves are concerned, they seem right, aside from the use of the evil French units of measure.