Vague question: what common medical conditions could arise from this change of habbits: from walking everyday to class, to driving every day to work?

It varies. If you are not getting any other forms of exercise you could put on more weight. Increasing your weight may put you at higher risk for hypertension, diabetes, cholesterol elevation, cancer, etc.. Of course there are many other factors besides exercise that affect your health as well.
Obesity and cad. Decrease exercise because of this change in habit can lead to increase weight thus eventually obesity/overweight. You are more likely than to develop high blood pressure, diabetes, obesity, and maybe sleep apnea.