My DC goes to SCD, but gets the school bus, so does not go on the train, but his long-term friend does (and then she walks from Chorleywood Station to SCD). My DC has informed me that a lot of the children take a shorter route walking from the station to the school, through the "cow fields" - fine if your DC is at one with nature, but not so good on wet days (and obviously there's a need to avoid the cow pats!
From what I can make out, the journey from Chorleywood Station to SCD is 1.1 miles, which apparently takes 21 minutes to walk (based on walking at 3 miles per hour). As you said, it does appear that part of it is uphill.
I don't personally know which bus services run around there, but perhaps you could try one or more of these suggestions:
1. ringing the reception at SCD and asking them if they know. Here's their number: 01923 284169
2. ringing Intalink 0300 123 4050 (they run a lot of bus services, I don't know if they run the service you would want, but its worth trying)
3. going on Intalink's website: http://www.intalink.org.uk
4. emailing Intalink: email@example.com
5. ringing Herts County Council Passenger Transport unit
I can't guarantee that you'll get an answer from the above, but I hope that the info is of help somehow and that hopefully it points you in the right direction.