Who doesn't advise it for U12s?
Also, if it's advice then it's just that - an opinion.
What matters more is your opinion about your child and ^your circumstances.
When my ds (who is organisationally challenged) started secondary, he walked to the CMs rather than home to an empty house for the first half term or so, until he'd got his head around all the new changes and then wanted to come home himself. My 2 dds just came home.
All these things though will depend on what time she will get in, what time the first parent will get in, what your neighbours are like (if they are home at that time), how confident she is, if she's used to letting herself into the house, if she's used to being on her own for a couple of hours, etc.,etc.
Often schools have after school clubs or areas students can go and sit and work in, but the issue then becomes they are travelling home on their own, later (and it soon gets dark in the Autumn) rather than in the light, surrounded by hundreds of others travelling at the same time.