Fortran was my second language (college) and back then it was one of the languages of choice for differential equations applications.

It is still being used in scientific computing and has been modernized substantially since the FORTRAN 77 you probably used. There are free compilers gfortran and g95 that can be tried.

Nowadays many engineers use Matlab, the full version of which is expensive. The student version for high school and college students is cheaper. Octave is a free alternative compatible with Matlab but does not have a GUI. Python with SciPy has many of the capabilities of Matlab and is what I'd recommend for students who want to program for scientific research.