strings as arrays (New Features)
Thinking about your request has opened up several little issues. BASIC-256 does not have the concept of a 'x' char like c does. What would the behavior a$ = "abcde" do? Insert replacing one char (at 6) or replace 5 chars starting at 6? I believe either way would be confusing.
I have looked at the MID statement in other BASICS and find it confusing but doable. Making it work like VB6 or GWBASIC would be very possible. Would that be a solution for you? http://www.divonasperi.it/divona/tam/tecnologia/dow-all/GW%20Basic%20%28inglese%29.pdf
Let me think about array indexing a string variable. It might not be as difficult as my first thought.
Would love to know more about how you are using this in your University. I have been teaching an intro class using BASIC-256 then moving to VB.NET for several years. It seems successful.