I was admitted to a school in France long-term student visa (multiple entry), but I had to drop out after 6 months. Now I have gone to Spain, another Schengen state, and see good schools where I would like to study. How do I proceed? If I am accepted by a school in Spain, can I change my visa?
Schengen long-term visas are issued by individual states and governed by the law of each state. Someone with a student visa for France who wants to go to school in Spain must apply to Spain for permission to reside and study in Spain.
You may be able to apply for a Spanish student visa at a Spanish consulate in France, or you may be able to proceed to Spain and apply directly for a residence permit there. Otherwise, you will probably have to go back to your home country to apply for a student visa there. To find out, you can start by asking at the Spanish consulate responsible for your place of residence. For example, if you are in the territory of the consulate in Toulouse, you can consult their pages on long-stay visas and on studying in Spain.