The students say they thought they had served enough detention time to be able to go, but someone allegedly falsified their detention records.
Randy Bui, 18, spent about $1,200 on the prom only to learn he's not allowed to go.
"I'm going to miss the prom. I am suspended today," he said.
Bui is among 20 to 30 students who received suspension letters on Thursday. Garden Grove Unified School District officials say the students, most of them seniors, are suspended for one to five days, including Saturday's prom.
"They're just saying that we had people clear our tardies and we all got suspended for it. And some of us didn't even do anything," said student Rian Logan.
All of the students we spoke with admit they had some detention for being late to school. But they all claimed they were serving detention or preparing to when they found out their tardies had been removed from their record.
"The girl told me my tardies were cleared. I'm thinking, 'Oh they got cleared by the school or whatever,'" said student Tyree Howard.
Howard says the girl who cleared his tardies without his permission was a student assistant.
District officials say several student assistants illegally changed the attendance records of the students. The district learned about it Wednesday afternoon, after an assistant principal had already given the go-ahead for the students to go to prom.
"About a week ago, we had to turn in a permission slip where the AP had to sign it. She had to go through all our tardies, all our absences and make sure we were OK to go. She signed it," said student Fernando Banuelos.
In a statement, the district said, "Students must clear attendance issues by serving detention before they are able to buy prom tickets. Because administrators were viewing illegally altered records, they signed-off on ticket purchases for students who had not actually cleared their attendance issues."
Some parents say they wish they would have found out sooner instead of two days before prom.
"It's cost a lot of money for prom. We can't return half the stuff that we got -- the dress, the shoes," said Robbie Munoz, a student's mother.