Unless there are plans to change it, the Philip Morant school day is out of step with the other two ? 8:15 - 3:00. So they should be in school/gone before your DDs are using the path. Some PM pupils using the buses from the bus park on the Colchs side may hang around after school as the buses dont go til later when the other two schools finish.
DD uses a bus from the bus park so no experience of going the other way.
Yes, Philip Morant school day starts earlier, they are in their first lesson at 08:35., but there will always be late ones. They finishes at 3.
I drop off my dd year 7 around 08:25-08:30 at the entrance of Norman fields, and she walks to Colchs by herself, no problems as such yet. Sometimes she arrives into school just before or just after the school bell, but on time for class registration. I see a few Colchs girls making their way through the field entrance during that time, so I assume its fine for them to walk that way.
At the end of the school day, I collect her from the school car park and have noticed St Benedicts finishes 5 mins earlier than Colchs. She walks out with the colchs crowd, so no problems there either.
Hope this ease your concerns.