PickACoolUserName it's a bit more complicated than that for all premises.
By that logic, my local Tesco would be allowing about 100 people in at a time, rather than 30.
Yes, lots of classrooms are tiny. Victorian buildings tend to have smaller rooms, as do the modern, purpose built schools.
Additionally, it's logistically impossible to have pupils sitting 50cm away from each wall and door as your working suggests. They need to be able to enter and exit the classroom easily (fire legislation) and see the whiteboard and teacher.
When they or the teacher moves around the class eg entering or exiting, they will need to be 1m away from all desks, so the desks effectively needing 2m between them.
The 1m also needs to be included in working out how pupils and staff will move around the building (some esp secondary schools have very narrow corridors), use toilets, enter and exit, eat lunch, have break time etc.
In primary, social distancing also needs to apply to drop off and picking up arrangements.
This is what is giving teaching staff a headache at the moment!