A couple of years ago I helped out in a local (French) school with children in the age group 7 to 11, just for one hour a week to improve their English.
Incidentally I am not, and never was, a school teacher and I'm doing this course because my French needs a lot of improvement (I communicate well but my grammar is appalling, probably as is my accent).
The method I used was to give a child a number either in French or English, between 1 and 999, and for them to respond in the other language. I would just ask around the class randomly so that no one was able to prepare in advance. This was just for 5 minutes or so at the end of the lesson. I did not pressurize them and I never let a child struggle but by the end of the year most of them could come straight back with the answer. In total they had about 3 hours over the year so if you can get someone to play the game with you I'm sure you will improve. The children looked forward to it and always made sure that the game wasn't forgotten.
If you can get someone to play the game with you it will improve your numbers very quickly.