Oakmeeds Community College in Burgess Hill has been placed into special measures.
Inspectors say pupils at the college, where GCSE results were below the national average in 2013, are not achieving their academic potential.
The school, which has more than 1,000 students, has been told to improve after a visit from inspectors on January 29 and 30.
In a letter to parents headteacher Colin Taylor said inspectors took a “harsh view” in the report, which is due to be published tomorrow.
Mr Taylor also invited parents to discuss the report at a meeting on April 2.
The letter read: “The team was impressed by the students who were in a safe environment and showed good behaviour, praising the positive attitudes students displayed towards their learning.
“However the inspection team judged that students were not seen to achieve to their academic potential.
“The inspectors acknowledged that following the 2013 GCSE results actions had been taken to raise standards but these have not yet had sufficient time to show the full impact and also needed to be more strongly and rigorously implemented.
“The governors and staff are fully committed in addressing the issues and action points raised in the report, and the local authority has expressed every confidence that with all the measures in place Oakmeeds will recover its 'good' rating by summer 2015.”
Mr Taylor added 93% of parents said their child was happy at the College, 89% stated their child was taught well and 91% believed their child made good progress at Oakmeeds.
He also said 89% of parents who completed the online survey said they would recommend Oakmeeds to another parent.
One parent, who wished to remain anonymous, said he “wasn't surprised in the slightest” at the news.
The father, whose child is about to start the second year of her GCSE's, said: “It's concerning and I'm not surprised in the slightest.
“Last year the school spent a lot of time trying to change the uniform, which cost parents double what the previous one cost, and also trying to change to academy status.“It seems the senior management have forgotten about the education and instead wasted a lot of valuable energy on other projects while forgetting about the kids.”