We don't have anything like a 'homecoming' event in the UK. Some schools may have a class or year 'reunion'. But they would be locally organised events on a small scale.
Our education system is quite different to the American system. We don't have 'high schools' and 'colleges' in the same format as you do. We don't have a 'graduation' either until you get a degree in univesity.
The UK education system is organised like this, (may be some local differences, like in Scotland):
under 4: Nursery School
5 - 6: Infant School
(some infant schools have a 'rising 5s' class for children under 5)
7 - 10: Junior School
(You can get combined infant and junior schools called primary schools)
11-13: Secondary School
14-16: College or Upper School
At 16 you take 'GCSE' exams. You can then either leave school and get a job or apprenticeship, stay on at school to take 'A' level exams (you need 'A' levels to get into university), leave school and do nothing (dropping out!), or go to a further education college to study a profession or skill (i.e. hair dressing and beauty, plumbing, building etc.)
19-xx: University