More than 100 people have been struck down by a gastro outbreak at the Sandown Park Greyhound Racing Club in Melbourne's south-east.

Teachers and students were among the 100 people who fell ill following a valedictory function for Wantirna Secondary College at the club's function centre last Wednesday.

Four people, including two teachers, needed hospital treatment for dehydration.

The club was closed for commercial cleaning the following day, but more people reported suffering vomiting and diarrhoea after attending the greyhound races on Friday night.