[hardwired]

Code:

File f1 = new File("students17.dat");
File f2 = new File("passing.dat");
File f3 = new File("honors.dat");

Files with a dat extension are often written and read with highly specialized DataOutput/InputStreams. We generally use a txt extension for reading with a FileReader. It may work; I haven't tried.
The error on line 30 suggests that the reader read something that came up as "null". If the data in the file is only a number on each line then your read method should work.
An impertinent suggestion is to move the sum variable up above the while loop. It gets reset/declared and assigned the value of zero in each trip throught the loop.

Code:

int sum = 0;
while((inString = inStream.readLine()) != null)
    System.out.println(inString);
    int rec = Integer.parseInt(inString);
//    int sum = 0;
    sum += rec;