The 15-year old pupil at the Ridgeway school was allegedly hit with a hammer as he left after classes just before 1600 GMT on Thursday.
The teenager, who has not been named, was taken to the Great Western Hospital where he is been treated for head injuries. His parents are at his side.
The incident happened as scores of children were leaving to go home.
A source said the boy had been held down and hit with a hammer by the family of another pupil at the school.
The eight men are being held at Gable Cross Police Station in Swindon.
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