FWIW, 20:00 and 19:38 would be very, very good for a 6th grade girl, I’m assuming most likely around age 12 now. You mention the girl goes 1:05 in 100 short course yards, therefore her ability is likely nowhere near the 1500 times and could take a very long time to get there. If a 12 year old girl were to go 1:05.00 in 100 long course meters free or 58:00 in 100 short course yards and she’s really more of an amazing endurance swimmer, she might be at those times today.

There were roughly 110 girls aged 11/12 that swam a 1500 LCM time at 20 minutes or under last year in USA. That being said, that event is rarely done in that age bracket.