Whilst it is hard for a parent to hear, most 12 year olds are delighted to get away from them, be it a school bus or, as you put it "normal" public transport. It is, well....normal...and to be advised! The sooner they learn to travel independently, the more confident they become with dealing with the odd occasion where the bus is late, or they miss the bus, and know they can wait for the the next one, or ring the school just to let them know they will be late. For every parent like you who is worried, there will be many more who are quite happy to pop their child on a bus on day one, a bus they may very well have not been on, because they trust that, along with the other kids on teh bus, they will get there. It's amazing bow older students just know to look out for younger students on those early days - one might imagine that they remember back to their early days...!

And, there is a long time till September - you can do the journey together for a day or so over the holidays - and even pop them on the bus and follow in the car to make sure they get off at the right stop etc, as practise, so by the time you get to Septemeber they are actually old hands at the journey. Far better that, and them being able to fully immerse themselves in school life than living in another town and having to whip out at the end of the school day...
Unless, of course, as Toadmum identifies, your child has some sort of special need that means they are unable to navigate transport on their own...?