Organised by the Ipswich group of Catholic schools, the Mass included representatives of 26 schools from Peterborough, Cambridge, Ipswich, Norwich, Swaffham, Bungay, Beccles, Newmarket, Bury St Edmunds, Lowestoft and Gorleston.
The Mass, which was led by Bishop Alan Hopes, also celebrated the Jubilee Year of Mercy and special candles were given out to each school to take back with them, representing unity across the Diocese.
In return, each school presented Bishop Alan with a book containing the names of all of their pupils and staff to be prayed for during the Mass.
Theresa Barker, head teacher of St Mark's Primary in Ipswich, and one of the organisers of the Mass, said: "It’s wonderful that children and staff representing all of the schools in the Diocese can come together in this Year of Mercy to celebrate the beginning of a new academic year.
"Even although almost 700 people have come here today, many more in our schools are with us in spirit and are named in the books that each school brought up at the offertory. And it’s fitting that, through the candles, each school has received today, we can all continue the togetherness and joy of this mass throughout the year ahead."
During his homily, Bishop Alan explained that the candles, which were given to each school, have a picture of Jesus Christ carrying a lost sheep over his shoulder.
"It reminds us of God's wonderful love and mercy which he shows to each one of us. Jesus goes out of His way to seek the one who has lost his way. He calls you and me to follow Him. Not because we are perfect but precisely because we are imperfect. He calls us because he knows he can change us and give us purpose in life ' and make a difference to the world in which we live."
Young cathedral choristers were joined by others from various Ipswich schools to form a special choir for the occasion, and other Ipswich pupils read the lessons and prayers.
After the Mass, everyone went outside into the sunny Cathedral Narthex gardens to enjoy a picnic lunch.
See below for a full picture gallery of the occasion.