Simplify the following text for a Grade 9 native English-speaking person.
Original: The highest seasonal average at the stadium since Ipswich turned professional was 26,431 in the 1976 â `` 77 season while Ipswich were playing in the First Division .
Simplified: The highest seasonal average at the stadium since Ipswich turned professional was 26,431 in the 1976 â `` 77 season .