I'm not too sure, I'm afraid, your best bet is to go onto the Herts council website and get the list of schools and then check the Ofsted reports which will be on the schools' websites.
Here is something quite unscientific...
primary-schools.findthebest.co.uk/d/a/St-Albans
However, I would caution 'picking' a school. It is not a case that you find a school that you like and apply. You have to wait for a school place to come up, and there will already be a list of people waiting for that place at the good schools.
Therefore you have to be living closer than anyone else and be ready to take up a place at sometimes one days notice.
So, say, you fancied the Abbey School, which I know is very good, you would have to move right next to it, send your child to whatever school you could get into, and wait for a place. I presume your child is in Y5, there is a strong chance that no place comes up for the next year and a half, so you would have to buy/rent in one of the most expensive parts of St A on the off chance that they would get in.
The sad fact is, that you don't really have a choice. There will be a list of schools with available places on the council website.
On the other hand, if you are keen to look at St Alban's, have you looked at St Columba's? It is a prep school as well as a senior school. I don't know much about the admissions, but if you can pay, you may have more luck. Ditto, the other boys prep schools in the area; Heath Mount, Lockers Park, etc.