Don't take the following as acurate - I have no inside knowledge of the marking process.
Firstly each test is marked and a 'Raw score' allocated to the paper. That's purely the number of correct questions. Those two figures are then passed to all the schools that the parents had ticked the box for on the back of the application form.
Each school will then analyise the cohort of children who wanted the results shared with them by age. This gives some kind of a weighting (or multiplier) unique to that school, which is applied to each childs raw score depending upon the month they were born in (and hence level of academic maturity), to ensure that very roughly
a child born in July gets the same weighted score as a child born in October if they are of equal intelligence (etc); in other words, to ensure that roughly 1/12th of each intake comes from each month.
Each school will have a different cohort - so children from the Forest may not tick for the Stroud schools due to distance, and likewise Fairford children not ticking for the Gloucester schools (though many parents will have done so out of pure inquisitiveness
). Therefore the weighted score for a child for one school can not be compared with the score they got for another school.
The weighted results are then put in some kind of a rank order and compared with the pass mark for that school. Each school will have many, many more children being deemed as 'selective' (ie pass) than they have places - every child with multiple ticks on the application form may have passed for several schools, but can only actually take up a place at one of them come September!
So you'll get a letter in a week or so with the results of the test, allowing you to fill in the secondary application form.
Note that the 'waiting list' is totally separate, and doesn't come into effect until after national allocation day (possibly even until after re-allocations).