Sensors to detect humans

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Im working on a project which involves detecting and counting the number of people who enter and leave a classroom. The sensor should detect if the person is entering or leaving the room.If two or more people enter or leave the room simultaneously at the same time walking side by side the sensor should be capable of detecting that as well. I'm looking for ideas to implement this and the right sensors i must use to achieve this. i request people who have experience in the field or who might have involved in projects similar to this to help me with their ideas.........