Here is the DfE's non-statutory advice 'Standards for School Premises'.
Government documents (advice, guidance and even secondary legislation) have not always represented the primary legislaion correctly, so may be worth checking out the regulations within the advice document.
www.gov.uk/guidance/standards-for-school-premises
"The Standards
Toilet and washing facilities
ISS Regulation 23A — (1) Subject to sub-paragraph (2), the standard in this paragraph is met if the proprietor ensures that—
a) suitable toilet and washing facilities are provided for the sole use of pupils;
b) separate toilet facilities for boys and girls aged 8 years or over are 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; and
c) suitablechangingaccommodationandshowersareprovidedforpupilsaged11 years or over at the start of the school year who receive physical education.
(2) Where separate facilities are provided under sub-paragraph (1) (a) for pupils who are disabled, they may also be used by other pupils, staff, supply staff, volunteers and visitors, whether or not they are disabled.
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.
(3) Where separate facilities are provided under paragraph (1) for pupils who are disabled, they may also be used by other pupils, teachers and others employed at the school, and visitors, whether or not they are disabled.
(4) Suitable changing accommodation and showers must be provided for pupils aged 11 years or over at the start of the school year who receive physical education.
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. 5
General planning
Toilet facilities need to be planned and designed so that:
a) hand washing facilities are provided within or in the immediate vicinity of every toilet;
b) the rooms containing them are adequately ventilated and lit;
c) they are located in areas around the school that provide easy access for pupils and allow for informal supervision by staff, without compromising pupils’ privacy.
Where there is unisex provision (under ISS 23A (1) (b) or SPR 4 (2)), the privacy of the occupant needs to be ensured and this will be achieved by, for example, having adequate enclosure and a full height door.
Facilities for disabled pupils
Each toilet for disabled pupils needs to contain one toilet and one washbasin (and possibly a shower or other wash down fitting) and have a door opening directly onto a circulation space that is not a staircase and which can be secured from the inside. Where possible, the number and location of accessible toilets will be sufficient to ensure a reasonable travel distance for users that does not involve changing floor levels 5.
Changing accommodation and showers for pupils
It is preferable for showers to be in areas separated from toilets and they need to provide adequate privacy. Consideration may also be given to providing changing rooms, with or without showers, at primary schools for pupils who need to wear sports kit for physical education, but this is not required under the regulations.
Toilets and washing facilities or staff
Toilets and washing facilities for staff6 may be also be used by visitors. They should be separate from those provided for pupils, except where they are designed for use by those who are disabled.
5 The requirements of Part M of the Building Regulations will be satisfied if any wheelchair user does not have to travel more than 40m horizontally to reach an accessible toilet. With young pupils the travel distance may need to be less.
6 Regulations 20 and 21 of the Workplace (Health, Safety and Welfare) Regulations 1992 set out the standards for staff toilets and washing facilities."