Replacing a string with a random number from vector

I have a dataframe which includes grades of students per course. These grades, however, are in A-F format and need to be transformed to numerical grades (10-1). To do so, I generated random numbers that represent these A-F values.

A <- rnorm(nrow(Student_Data), 9.45, 0.2)
B <- rnorm(nrow(Student_Data), 7.95, 0.2)
C <- rnorm(nrow(Student_Data), 6.25, 0.2)
D <- rnorm(nrow(Student_Data), 4.75, 0.2)
F <- rnorm(nrow(Student_Data), 2, 0.2)

I also created a function which allows to replace the letters with numbers

courseGradesNumeric <- data.frame(lapply(courseGrades, function(x) {gsub("A", sample(A, 1), gsub("B", sample(B, 1), gsub("C", sample(C, 1), gsub("D", sample(D, 1), gsub("F", sample(F, 1), x)))))}))

This works quite well but the problem is that if there is an "A" (or any other letter) in a column, then this A in this specific column is replaced by a random number from vector A that is the same across the entire column.

To illustrate:

Current dataframe (ignore the NA's for now)

Student_ID       ABC1000_Grade   ABC1003_Grade 
1    9000006           A              B          
2    9000014           A              A          
3    9000028           B              C          
4    9000045          <NA>           <NA>          
5    9000080           C             <NA>          
6    9000091          <NA>           <NA> 

The problem:

Student_ID       ABC1000_Grade   ABC1003_Grade 
1    9000006        9.335523      8.231295          
2    9000014        9.335523      9.462468          
3    9000028        7.972959      6.394259          
4    9000045          <NA>           <NA>          
5    9000080        6.257297         <NA>          
6    9000091          <NA>           <NA> 

In the ABC1000_Grade column, the A was replaced by the same random number that was generated in an earlier step.

How can I make sure that all replaced values are different random numbers? Thus, the preferred result should be:

Student_ID       ABC1000_Grade   ABC1003_Grade 
1    9000006        9.510445      8.231295          
2    9000014        9.335523      9.462468          
3    9000028        7.972959      6.394259          
4    9000045          <NA>           <NA>          
5    9000080        6.257297         <NA>          
6    9000091          <NA>           <NA>