The teachers, pupils and their parents who were waiting to collect them, found the stowaways when they arrived home from the three-day trip.
The men, thought to be in their early 20s, were found hiding among the luggage when the hold was opened after the journey back to the UK.
One mum waiting to collect her child said the incident at Hounsdown School in Totton, Hants, was “horrendous”.
One of the men tried to run off but was stopped by parents and one child’s luggage was said to be covered in urine.
‘It was horrendous’
The coach had been carrying Year 9 and 10 pupils who had just returned from a three-day school trip to Boulogne University which is about 20 miles south of Calais.
The teachers and pupils had travelled home on the coach via the Eurotunnel at Calais, returning to Totton, near Southampton, on Saturday evening.
“It was horrendous,” one mother said.
Describing the moment the men were found, she said: “[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.”
Following the discovery, she panicked and went to grab her 14-year-old son’s baggage from the coach.
She said: “The only way I can describe it is panic mode.
“My one aim was to get my child off the bus, and once I had I went back as I couldn’t handle seeing kids being asked to take their baggage out of the hold.
“I just said ‘excuse-moi’ to the gent and leaned over him to get the rest of the bags out as my son’s was tucked underneath him.”
‘My son is so upset’
Posting on social media, one parent said: “Just picked my daughter up from her school trip to France. The coach driver opened up the bottom to get the suitcases and found three immigrants sat in there.”
Another parent said: “My son is so upset, bless him, he’s told me to cancel him going away next year with the school and has said he’d rather wait until he can pay to go when he’s older.
”Poor little… after such an incredible trip too! The teachers couldn’t have done a better job the whole way through, I really do feel for them right now.”
Hampshire police confirmed that officers attended but no arrests were made.
A force spokesman said: “We were called just before 05:15pm on Saturday, February 10, 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 Home Office and Hounsdown School have been contacted for comment.
In 2014, an immigrant clung to the bottom of a school bus for more than 100 miles after it made its way back to the UK from a day trip to France
The bus was packed with pupils from a school in Ilford, northeast London, when the driver heard noises coming from underneath the coach.
The driver drove the coach to Ilford Police Station, where officers found a 16-year-old from Sudan “attached to the chassis”.