First we can point that in French with capital letters, the accent is not compulsory, therefore
Ecole; the word would still pronounce the same.
I don't think there is a real strict agreement for acronyms (or initialisms) and you are free to use them as you wish but for moderation the general rules are:
- we always use capital letters
- we never retain the accents
Any large instance/company will follow as much as possible these rules.
For the pronunciation it is the same, there isn't a real strict agreement but we pronounce the letters as they are written. the motivation is that acronyms are not meant to be used for representing each word but to give a shorter name to a fixed redundant set of words.
therefore ENS (for École Normale Supérieure), will pronounce as /ə/ (the French letter "e")
This said if you pronounce /e/ or even write
ÉNS it is not incorrect, but those who already use the acronym for a long time may feel uncomfortable. Also those attached to rules may bring the ones I described above.