Geoffrey Robertson QC told an independent schools tribunal that closing the 78-year-old independent school would be an "act of educational vandalism".
The school's proprietor, Zoe Readhead, is appealing against a notice of complaint by Education and Employment Secretary David Blunkett, issued after an OFSTED report found serious weaknesses.
The inspectors reported that accommodation at the boarding school in Leiston was inadequate and that children's non-attendance of lessons undermined their learning. But Mr Robertson, acting for Ms Readhead, said the children's freedm to decide whether to attend lessons was not negotiable.
Mr Robertson claimed that the report was based "on the briefest of snap shots" of morning class. Analysis by independent experts had "left no doubt whatsoever that the work of OFSTED was incompetent, biased and distorted".
Inspectors had taken no account of the voluntary learning that went on outside lessons, had not mentioned small class sizes and had taken no account of the achievements of ex-pupils.
Alison Foster, counsel for Mr Blunkett, told the tribunal that the Secretary of State said the school should work to make lessons attractive, to encourage attendance without compelling it.
The tribunal, which began on Monday, is expected to conclude next week, though judgment will be reserved.