Over large populations, high school grades are predictive of college grades. But for an individual student, there is enough individual variation that it should not be surprising if a student earned much better or much worse grades in college than in high school.
But note that college grades generally are likely to be lower than high school grades. At the most selective colleges, A high school students will be distributed across the A and B range in college (with some C students). At moderately selective colleges, A and B high school students will be distributed across the A through C range in college.