Indeed. She may not have helped to commit the crime, but by not reporting it, she was obstructing justice and abetting his evasion of arrest.
Also, there have been a couple of episodes of
L&O: UK
in which a young child was a murderer and was tried as an adult, so I would think that a high-schooler would be treated the same.
(Response notification is off.)

In the US, the age at which one can be tried as an adult varies by state and sometimes by crime and what the possible punishment may be.
A high schooler accused of being an accessory to murder will be tried as an adult even if they are of low mental functioning, particularly in the southern parts of the US.

Thanks for your reply.
Jess is a minor, not an adult, so how would she be treated?
God is subtle, but He is not malicious. (Albert Einstein)

If a murderer comes to you and tells you what he/she did, you now know of the crime. If you know the police are looking for the murderer and you don't tell the police who it is or where you think the murderer may be, you're withholding information from the police.