If the dogs are correct and the scent stops at the end of the driveway, then i think whatever happened, happened here.
So that means 2 things. Either she got a lift in a car from someone she knows, or she was abducted by someone with a car or got in a cab.
If she was picked up by someone she knows, I would imagine that the police would already know who this is. She would have had to arrange this by text or email or phone call, so there should be evidence of that on her computer or phone. Which makes me think that this isn't the case. You wouldn't be stupid enough to arrange to meet someone to abduct them and leave any evidence of that. (of course I could be wrong). Which makes me think that this scenario is highly unlikely.
Scenario 2, someone abducted her, very close to home. Very risky but we have heard so many cases where people are abducted in broad daylight. She was in a routine everyday. She left for the bus same time, following the same route. If her mother was in a similar routine then it would be easy for someone to find this out. It was early in the morning, not sure if it was dark, maybe not a lot of activity happening in the street she lived at that time. People still in bed, or getting on with morning tasks - curtains closed. Again easy for someone to find out. Then the mobile phone was found near to her home, which suggests that someone dumped this very soon after she went missing. Which makes sense if your abducting someone to get rid of the one thing that could track them.
Then the bag was found, no teenager running away would ditch their phone or their bag and run off with no money no clothes. did she have any reason to? appears not. Are the clothes relevant? maybe - maybe not. May have just been a change depending what classes she had that day. Was maybe going somewhere after school? Again if she had packed some clothes to sneak off to meet someone - there would be a record of this somewhere on the laptop or phone.
She didn't walk into the field and put it there as the dogs found no scent around the phone (or the bag). So either someone put it there or she or someone threw it into the field. I think this was thrown in the field by someone to get rid of evidence.
Anyway, just thinking out loud