Have you ever wondered why Snape (despite giving out multiple detentions to Harry) did not deduct any house points from Gryffindor after Harry nearly killed Malfoy with the Sectumsempra Curse? Normally Snape would be more than willing to use any excuse to take scores of points off Gryffindor, especially when it involves Harry. This would have been his golden opportunity to take off 200 points (or more) from Gryffindor (and he would be justified in doing so).
But i guess he did try to ensure Gryffindor wont win the Quiddich Cup by keeping Harry in detention throughout the match, but to no avail.