If you have help, lots of help; you could break them down into several different skill zones holding one practice for all of them three times a week. Another idea is to take the pitchers and catchers and have them come 30 minutes earlier than the others. Stagger practice times for the infield and outfield too; that way you are able to concentrate on fewer players each hour for defense. Another day could be scrimmage day, it would allow teaching moments that happen in a game so your young ladies are able to concentrate on playing the game when it comes around.
The trouble with no time and no place is you have A LOT of kids to handle and if you don't have enough help to keep them active it doesn't "feel" like a productive practice. You've got a popular problem...hope some other ideas come up.
I agree with LISA , the groups are great. they will learn the basic's better that way. also if the jv players are pretty good , mix them in some time with the varsity . like if if have two are three that say ss put them out there side by side and let them take turns fielding balls that you hit. tell the varsity players to give them pointers when thay make a mistake.