"I'm gutted. When I was told I was going to be excluded I was shocked, I couldn't speak," said Miss McDonald.
"I've always wanted to be a teacher but this makes me think it's not something I want to do, if this can happen to an innocent person. It definitely was not me.
"I loved school and the environment, I thought all the teachers were really friendly, understanding and there to help."
Her father, Peter McDonald, added that the two-hour polygraph test found that his daughter was 97 per cent likely to be telling the truth.
He said: "Alexandra is really stressed. She has never been in any trouble before. This is the harshest punishment before expulsion.
"The report from the lie detector is 98 per cent accurate, DNA evidence is 99 per cent, so it does prove she is telling the truth.
"When I heard why she was excluded I had to get the school to repeat it twice, I couldn't believe it.
"I asked for proof and was told it was on probability, a teacher saw them giggling in the same corridor but they didn't see them with the nuts.
Mr McDonald, from Maidstone, has said he will now take action in an attempt to get his daughter back into lessons.
"The exclusion will be appealed to the school governors, so I got the lie detector done to prove her innocence.
"It's so frustrating," he added.
Although teachers at Rochester Grammar School are yet to change their mind, based on the findings of Miss McDonald's lie detector test, Miss Colley is considering following suit.
Her mother, Michelle Colley, said: "It's just really disappointing. It was Maddie's 17th birthday on her first day of being excluded so that put a dampener on things.
"We've tried to convince the school but they've made their decision based on probability and they won't listen to any other argument.
"I'm hoping they will reconsider on the new evidence. I don't think we would go down the lie detector route but Maddie says she would take a test too.
"This will go on her permanent record if it doesn't get sorted. I fully believe her, if it was a practical joke she would have admitted it by now."
Describing what happened as "an extremely dangerous incident that could have had fatal consequences", a spokesman for Rochester Grammar School said: "Following an internal investigation, we took the decision to temporarily exclude two students.
"We make no apologies and are surprised a parent of one of the students does not see the seriousness and is contesting the decision to exclude.
"The school has been nut-free for over 20 years, and information is sent to every parent on their child starting at the school to confirm this. In addition, students are reminded of this at the beginning of each academic year, and it is written in their starter booklets."