One more thought - I knew some students that wouldn’t even bother to show up for the first week or two of classes. It wasn’t many, but there was always about 5% in each class that you’d see for the first time 2 or more weeks after the course started. Some may have just been waiting to get into a full class, but some were enrolled from the beginning and just decided to start showing up around week 2-3.
It was more noticeable in small classes with under 30, but if she was in intro classes with 50-100+ students, this would be very easy for her to get away with without the professor even knowing for weeks on end.
It was very unusual for students in 1961 to skip the first weeks of classes. The professors then were not the professors of the late60’s to now. The attendance rules would have been strict for at least one of the classes.