This is true. However, self-employed people are taxed far less on the same earnings. Also the number of hours that teachers actually work should be considered to calculate their true hourly rate.
Say for example if a teacher's salary is £35,000. Pension contributions are 9% from the teacher and 24% from the employer so 33%. But of course average salary DB schemes provide roughly 1.5x the benefits for the same contributions as a DC scheme due to the favourable tax treatment and index linking and the risk being borne by the employer not employee, so the rough salary required to replace the pension benefits through a DC scheme without the teacher decreasing their taxable pay is (24 x 1.5) + (9 x 0.5) = 40.5% contributions (on top of the 9% they already pay themselves from base salary remaining unchanged) so a total salary of £35,000 x 1.45 = £50,750
With 39 weeks per year of work if the teacher is in reality working a 60 hour week their hourly rate of pay is under £22, including being able to replace their pension benefits privately through a DC scheme and still take 13 weeks per year off! Their real hourly rate of pay is even lower if they also do some work in school holidays, which most say they do.
This means if they can secure an employed role at £22 per hour they will likely be able to ensure a similar net pay and pension outcome by working a more manageable 45 hours per week and having 6 weeks of annual leave (because holidays are paid): £50,750 / 52 / 45 = £21.68 per hour.
If self-employed 45 hours per week with 6 weeks of (unpaid) holiday they would in theory need to earn £25 per hour however, due to the hugely favourable tax treatment for self-employment, it would likely actually be a lower hourly rate required than for employed work taxed via PAYE to achieve the same net earnings.