For the poster upthread who asked why it is harder to carry the bag to school and back than to pick it up on the way home, I'll try and explain here. Apologies if I am being long-winded or telling you things you already know.
Working with a guide dog is quite a complicated business. You're not just holding a handle and being towed along to where you want to go. It is a business of continuous communication. As I hold the handle to the harness, I'm feeling information from my dog about where to go, and about how he's feeling/behaving. I can tell when he's a bit uncertain, or distracted or bothered by something. I can tell if he's sniffing about, or uncomfortable. Simultaneously, through the handle, he is getting information from me, about how comfortable I am, how fast I want to go, and whether I am uncertain/distracted/bothered. Or whether I've dropped the handle completely (signal to him to stop).
There is also a lead connected to my dog's collar, which I also hold. Mostly I am holding it in the same hand (left) as the harness handle, but at times I need to transfer it to my right hand (to signal my dog to turn right, or to slow down, or to calm him if he is nervous or distracted - he knows which of these things I want to do depending on the simultaneous voice command or foot position.
I also do hand signals for different commands (forward, right/left turns etc.) Again, this is simultaneous with hand positions and voice commands.
All of this takes a lot of continuous, sustained concentration on both our parts.
My dog doesn't know where I want to go, he's not a sat nav system. He might guess (or have his own opinion where I should go, i.e. park!) But I have to give him the commands at every point about when to turn. I have to concentrate hard to make sure I know where we are (how many roads we've crossed / turns we've taken etc.) I also have to decide when to cross roads (dogs cannot judge traffic) by using the remaining sight I have and listening. My dog is responsible for staying in a straight line as we cross the road and meeting the kerb straight on. I have to concentrate to make sure that I can feel if he starts veering off, and straighten him (voice command/hand signal/taking lead in right hand etc.)
All this means that tying poo bags to the harness or the lead are a no-no. The feel of something attached to the harness or lead would bother my dog and distract him from working. Similarly, me holding a poo bag in my right hand - seeing it swing about in the corner of his hand distracts him (I can't carry anything in that hand) and potentially prevents him from following signals and commands. The awkward feel of having to hold something there it would also distract me and make my movements more awkward.
On the way to school, I have my young DC with me and am responsible for their safety as well as mine and my dog's. I can't see them alongside me, so I have to maintain conversation with them in order to keep track of where they are at all times. It takes a lot of mental energy to do this as well as keep the communication channels open between me and my dog. I have inadvertently trained my DC to follow the same commands as my dog, which helps! (Stand and Wait / Forward / Stay etc!) 
On the way home, when it is just me, I am more able to manage the awkwardness of carrying something, and it is for a shorter time, as well. Also, my dog tends to know he is on the way home, and is more focused and needs less direction than on the way to school (which is also the same route to many other places, including the park!).
The only solution that's been offered on this thread which would work for me is to put the poo bag in my rucksack/the DCs school bags and carry it into the playground that way. This is what I am really unwilling to do, because it just feels wrong to me to take dog poo into school, and it makes me feel disgusting, and I would feel that other parents are shunning me even more than they do already. I would much rather leave it tucked away against the side of the path for 20 mins until I return.
This is only likely to happen a handful of times a year, I think maybe once every 2-3 months at most, because my dog is trained to do his business at home before we leave and I'm rigorous about his routine, so it is increasingly rare for him to need to go elsewhere.
However, I can see that the majority of posters on this thread feel it's unreasonable, so I will have to think about that, as I am already suffering a lot of general anxiety about the general public's attitude towards me and my dog.
Thanks, and particular thanks to the posters who responded empathetically, which was very kind of you.