The sound of a bell in a dream is usually a warning about something. Traditionally, the tolling of a bell is associated with death, so the tolling of a bell in a dream can be a warning that something bad may happen if you do not take care to prevent it.
A bell can also represent your conscience. Hearing a bell in your dream can mean that you are feeling guilty about something.
Church bells in a dream can represent tradition or spirituality.
If you dream that you hear a school bell, you could be thinking about your past. The sound of a school bell can also mean that there is something that you need to learn.