OK, so looking at this in detail...
A traffic order I can find - which may not be current - says the restriction only applies in term time.
Assuming this is West Lodge Primary School, its website gives end times for children for the term before the afternoon restriction time on 18 July.

LONDON BOROUGH OF HARROW
The Harrow (Prohibition of Stopping Outside Schools) (Amendment No. 1) Traffic Order 1998 (The Harrow (Waiting and Loading Restriction) (Amendment No. 103) Traffic Order 1998
Notice is hereby given that the Council of the London Borough of Harrow, on 8th June 1998, made the above mentioned Orders under sections 6, 124 and Part IV of Schedule 9 to the Road Traffic Regulation Act 1984, as amended by the Local Government Act 1985 and all other enabling powers.
2. The general effect of the (Prohibiton of Stopping Outside Schools) (Amendment) Order will be to amend The Harrow (Prohibition of Stopping Outside Schools) Traffic Order 1997, so that in the length of street specified in the Schedule to this notice between 8.30 a.m. to 9.30 a.m. and 3 p.m. to 4.30 p.m. Mondays to Fridays
during school term times would be prohibited.
3. The general effect of the (Waiting and Loading Restriction) (Amendment) Order will be to amend The Harrow (Waiting and Loading Restriction) Traffic Order 1978, so that the waiting restrictions in the section of West End Lane listed in the Schedule to this notice would be the same as the restrictions for the (Prohibition of Stopping Outside Schools) Traffic Order rather than the Pinner Controlled Parking Zone time of Monday to Friday 11 a.m. to 12 noon.
4. The prohibitions referred to in paragraph 2 and 3 above in respect of anything done at the direction of a police constable in uniform or with the permission of a parking attendant and the carrying out of statutory powers or duties. However the restrictions listed in paragraph 2 would not allow the picking up or setting down of passengers and “Orange Badge” holders would NOT be exempt.
5. Copies of the Orders, which will come into operation on 15th June 1998, The Harrow (Prohibition of Stopping Outside Schools) Traffic Order 1997 and of the Harrow (Waiting and Loading Restriction) Order 1978 (and of the Orders which have previously amended that Order) and documents giving more particulars of the Order are available for inspection during normal office hours on Mondays to Fridays inclusive until the end of 6 weeks from the date on which the Order was made in Third Floor Reception, Department of Environmental Services, Civic Centre, Station Road, Harrow.
6. If any person wishes to question the validity of the Orders or of their provisions on the grounds that they are not within the powers conferred by the Road Traffic Regulation Act 1984, or that any requirement of the Act or of any Instrument made under the Act has not been complied with, that person may, within 6 weeks of the date on which the Orders were made, apply for the purpose to the High Court. S. J. Swain, Transportation Manager, Environment, Planning and Transportation. Department of Environmental Services, PO Box 37, Civic Centre, Station Road, Harrow HA1 2UY.
Schedule
Vehicles not to stop on School Markings when the Sign is displayed Mondays to Fridays between 8.30 a.m. and 9.30 a.m. and between 3 p.m. and 4.30 p.m.
School
West Lodge School, Pinner
Road
West End Lane, between a point 8.60 metres south of a point opposite the southernmost wall of No. 77 West End Lane and a point 3.60 metres south of a point opposite the southern boundary of Pinner Memorial Park.
11th June 1998.
https://www.thegazette.co.uk/notice/L-55152-493