Well you first read the file recursive record by record witch tells me that the file is a file of type TStudent
and on the end You try to put the whole array by one time to it
try this

assign(StudentFile,'Student.dat');
rewrite(StudentFile);

i:=0;
repeat 
Write (StudentFile, Students[i]);
inc(i);
until i>NumStudents;
Closefile(StudentFile); 

or You can try this two

Write (StudentFile, Students);
{without the [i]}

and if needed the count of TStudent records to write

Write (StudentFile, Students,NumStudents);