Statistics released to the Scottish Lib Dems through Freedom of Information requests from 31 of Scotland's 32 councils, show 310,739 sick days were taken among teachers, support staff and nursery staff between April 2021 and October last year.
Of those sick days, 202,128 were taken in 2021/22, while the remaining 108,611 were taken in the first half of 2022/23.
Scottish Lib Dem education spokesman Willie Rennie said staff were being "pushed to their absolute limit", adding: "Over the past years, teachers, support staff and nursery staff have all seen class sizes rise and classroom support plummet.
"It is little wonder that they are now suffering from such poor mental health. The impact on staff and the consequent disruption for children and pupils should make the Government sit up and pay attention.
"It comes at a time when the poverty-related attainment gap has remained broadly the same and worsened in certain areas.
"Scottish Liberal Democrats have been long-standing advocates for education. To get Scottish education back on track, we need to get the basics right.
"That means reviewing pay and conditions for staff in the education sector, creating more time for lesson planning and cutting class sizes so that pupils get the support they deserve.
"That's how we tackle Scotland's pressure cooker classrooms."
Teachers are locked in talks with the Scottish Government and councils over a pay deal, with a number of school days already lost to strike action. The Scottish Secondary Teachers' Association announced on Monday plans for two further strike days in February and March.
Scotland’s biggest teaching union has claimed the Scottish Government and councils “have little or no interest” in finding the funding required to end a pay dispute.
Following a meeting with the Scottish Negotiating Committee for Teachers (SNCT) – the body tasked with coming up with teacher pay deals made up of the Scottish Government, Cosla and teaching unions – Des Morris, the salaries convener at the EIS, said: “It is becoming increasingly clear that both the Scottish Government and Cosla have little or no interest in finding the modest additional funding that could bring a new offer to the table to potentially end this pay dispute.