The UK government guidance on toilets in schools, from page 5
SPRs Regulation 4 — (1) Subject to paragraph (3), suitable toilet and washing
facilities must be provided for the sole use of pupils.
(2) Separate toilet facilities for boys and girls aged 8 years or over must be provided except where the toilet facility is provided in a room that can be secured from the inside and that is intended for use by one pupil at a time.
I would argue that if they are cubicles in a line that is not an toilet but a bank of cubicles so not intended for one at a time. Also as the sinks are outside the toilet is not self-contained and not for one at a time.
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/410294/Advice_on_standards_for_school_premises.pdf
The ratio of provision might be useful, as the sinks are outside of the cubicles it counts as shared sinks:
Numbers of fittings - The regulations do not set the minimum number of fittings to be provided in relation to the ages and numbers of pupils. Generally the needs of younger pupils are likely to be
greater than those for older ones. So, for example, a provision of one toilet and washbasin for every ten pupils under 5 years old would be adequate, while that ratio could be doubled for pupils aged 5-11 to one toilet and washbasin for every 20 pupils.
For pupils over 11 one toilet per 20 pupils would be sufficient, but there is scope to reduce the number of washbasins where the washing facilities are shared.
Did a newsletter go out to inform parents? As a secondary school I'm sure parents of girls who will be starting their periods might have something to say about this.
This might also be useful from page 12.
www.schooltoilets.co.uk/wp-content/uploads/2019/02/CubicleCentre-SchoolGuide.pdf