Officers were called to Hounsdown School in Totton on Saturday evening following reports that two young men were found lying inside the hold of the vehicle.
Hampshire Police said the pair were “possibly illegal migrants”, but the Home Office has yet not confirmed whether the men have been detained. No arrests have been made.
Tabi, who had been at the school to collect her daughter, told the BBC: “My daughter came running over and she said 'there's people sat in where the suitcases are'.
“Obviously it's quite concerning because there were 36 children on the coach and no one was aware that they were there.”
The coach had been carrying Year 9 and 10 pupils back from a three-day trip to Boulogne University, located around 20 miles south of Calais.
Another parent told the Daily Mail: “[My son's] luggage was covered in urine and his belongings crumpled by one of the men being on top of it.
“I asked them (in French) if they spoke English or French and they replied French, I was about to ask them where they'd come from but got ushered away by the teachers.”
The latest Government figures show that there were 52,530 irregular migrants detected entering the UK in the year ending June 2023, up 17 per cent on the same period the year before. Of that, 85 per cent arrived via small boats.
Separate figures show that a total of 1,506 migrants have made the dangerous crossing of the English Channel so far this year, which is down from the 2,072 by the same point in 2023 but up from 1,339 in 2022.
Prime Minister Rishi Sunak has made “stopping the boats” a key pledge of his leadership as the country approaches a general election expected later this year.
A spokesperson for Hampshire and Isle of Wight Constabulary said: “We were called just before 17:15 GMT on Saturday to reports that two people, who were possibly illegal immigrants, had been found at Hounsdown School in Totton. Officers have attended. No arrests have been made.”
The Standard has contacted the Home Office and Hounsdown School for comment.