The start and finish times of the school day are decided by the governing body of each school, as is how the time in between is organised.
Mornings and afternoons always begin with a short registration period, allowing teachers to check that children are meeting the legal requirement to be in school. There are also always breaks at mid-morning and for lunch, but when they are and how long they last varies.
The majority of the day, of course, is devoted to lesson time. Again, the number of lessons in a day and how long they last is up to the individual school. For instance, some schools have 10 half-hour lessons in a day, while others have six lasting 50 minutes each.
The end or beginning of a lesson is often marked by a bell or some other kind of alert, but this isn't always the case, so time keeping is something that all teachers should be good at!
If you become a primary school teacher you will teach the same class in the same classroom throughout the day. In secondary schools, you will normally have your own classroom and various groups of children will come to you as determined by the timetable.