At the moment, Cainscross Road has a lot of roadworks where water mains are being replaced. Even at off-peak times, there are long traffic jams. These are due to continue for some months.
My experience of trying to drive along the road when it happened to be 11+ day is that it is somewhat chaotic at drop-off & collection time, without the added hassle of traffic lights.
Rather that get stressed in a traffic jam, I would strongly suggest that parents consider parking at either the Cheapside car park (that's the 'long term' car park, south side of the station, just past the Hill Paul 'tower block' - not the Railway car park) if coming from the East (Cirencester / Swindon), or the Cainscross car park behind the chip shop / chinese, if coming from the West. Both are a 10 min walk from the schools, which will also give your child a little exercise / fresh air.
If you're from outside the area, whilst the children are beavering away, you'll find Stroud has plenty of cafes & shops - there's a new cafe by the Wallbridge lock on the canal (by the canal visitor centre) which does decent coffee & cakes, and is where your children may end up after school if they pass the exam. Well, my Y9 DD goes there with friends (and at the moment, there's a web cam on the canal site so you can see more or less who'se going in there).