Different atmospheric models available for personal usage? We are two high schoolers who would like to embark on a project to simulate the atmospheres of exoplanets.

More specifically, we would like to change the atmospheric composition of such planets. We have seen various climate models, such as CESM and PlaSim. Unfortunately, CESM appears to be quite complex, while the latter option doesn't have the resources we would need.

Are there any other simulators that may consistently do the same thing, but need lower computer specifications? Otherwise, are there any resources to become familiar with CESM at a high school level?

Furthermore, what would be the required specifications for running CESM on a fairly consistent basis? I have heard that you would need a cluster, but we currently do not have access to one. We may be able to rent one out, but we'd rather be familiar with CESM before we do.

Thank you!

EDIT: (for clarification) I know a bit of FORTRAN, and have access to help from a Physics instructor. Although I understand that it may be out of the reach of two high school students, is there any sort of "crash course", one could recommend on more advanced astronomical physics?

Furthermore, we are interested in a rocky exoplanet, and potentially have access to a cluster/help from a professor.

Thank you once again!
Unfortunately, I don't think such a numerical model exists that would be suitable for high school students' understanding. The simplest climate model around is a zero-dimensional (energy balance) climate model, but a background on mathematics, and programming may be necessary. You can find one here. Wikipedia has one listed. Michael Mann has the source code for such a model, but it is written in MATLAB (you may be able to run it in OCTAVE, since MATLAB is licensed). There are likely many more than just these, considering multiple layers, volcanic eruptions, etc.

The code for the models listed are simplified models that do not consider things like clouds, aerosols, eddy diffusion, etc. This is where Global Climate Models (GCMs) differ. They must consider how fine details, some of which aren't considered by weather models, affect the climate. Therefore, they are usually complicated. Most GCMs I have seen are written in FORTRAN, which is an antiquated language, but computationally efficient. This makes even easy concepts obtuse.

While I have not dealt specifically with CESM, I can infer some things about it. By definition, CESM is an earth systems model, not an exoplanet model. You may be able to adapt some of the parameters to other planets, but it becomes too difficult for just one person to do (because of FORTRAN). There are models for other places, such as GCMs for Mars, but they are difficult to run and not necessarily accurate. Clusters are needed due to memory limitations, and how long they run for.