This reading is actually supported by a passage in The Voyage of the Dawn Treader (ch. 1):
Grown-ups thought [Susan] the pretty one of the family and she was no good at school work (though otherwise very old for her age) and Mother said she 'would get far more out of a trip to America than the youngsters.'Of course, we do know from Prince Caspian (ch. 3) that Susan won swimming prizes at school, so there's that.
* * *Coda - Approximate dates of composition and publication (courtesy of Joel Heck's chronology):
- Prince Caspian - finished by Dec 1949 and given to RGL to read, with comments back on Dec 31; in typescript by end of Feb 1950. Published Oct 1951.
- The Voyage of the Dawn Treader - ready for RGL to read by end of Feb 1950. Published Sept 1952.
- The Last Battle - Lewis is "attempting to complete" it in Dec 1952; finished by Mar 1953. Published Mar 1956.