Hello,
Please be informed that there are one languages with 3 different names in 3 countries:
1. Persian is official language in Iran (Farsi):
2. Dari is one of Official languages in Afghanistan. Another official language in AFG is Pashtoo. Pashtoo is very difficult and completely different from Dari. A difference between Dari and Persian is not big, difference is accent only.
3. Tajik is official language of Tajikistan and much different from Persian and Dari. People of Iran, Afghanistan and Tajikistan understand each other very well and they have no problem.
We can say that these 3 languages are like German in Germany, Austria and Switzerland.
In fact you do not need for Dari if you work for International organisations. People who work for International organisations speak very good English.
If you wish learn Dari you can do that. Visiting Schools not possible. As a foreigner you would be not allowed. But you can find a good teachers very easy and not so expensive.
I speak Tajik and Dari fluent. With Iranians I speak in Dari and they speak Persian (Farsi) and we understand each other 100%.
Hope this information helps you.
Best regards
Fozil