Dustin being 3 years younger had nothing to do with anything. He was a scrawny, socially retarded geek. No wonder the other kids in the cast avoided him.

to be only known as the goof and the one perpetually in girls clothing, i'd get pissed too!
but although Dustin realized his character and subsequently him were considered a farce, comedic filler, he should have gotten upset with the people who had made him that way. the writers/producers and even the audience were who wrote Screech as the bumbling genius/idiot we all know him.
i'd be pissed too. but I would get some psychological help.
Oh God. Fortune
vomits
on my eiderdown once more.

Now they learned a bad lesson for being both ableists and ageists at the same time on him, especially that entire original SBTB crewmember group, except for Peter Engel.

Oh yeah I heard about the other actors not hanging around with Dustin because he was quite a bit younger than them, they totally made Screech act like he was mentally impaired in the later episodes

It is Dustin's Screech role that has a disability. But as a matter of fact, the entire original SBTB crewmember group, except for Peter Engel, might be considered ageists, besides ableists, at the same time on Dustin completely also. Mario and Dennis should both know about this along the way, either.
Ageism is stereotyping and discriminating against individuals or groups on the basis of their age (either old or young). It can be applied in this original SBTB class series project as worse 1.) as ableism and 2.) than both racism and genderism.

Does Dustin have a disability?

An ableist is a person who discriminates and social-prejudices against people with disabilities.
Ableism characterizes persons as defined by their disabilities and as inferior to the non-disabled and it is also considered worse than both racism and genderism.
Peter Engel isn't considered an ableist because he's the producer who can handle and get along with all of these teens in all types, especially with all these six teens, including Dustin/Screech, and that principal altogether in this original SBTB class series, as revealed that he should not be blamed anymore.
The rest of the original SBTB crewmember group are considered ableists because they are more talented on focusing on more-talented and more-publicized originals people and places to earn more money of this original SBTB class series, which are these permanent ableists:

• Mark/Zack
• Mario/Slater
• Tiffani/Kelly
• Elizabeth/Jessie
• Lark/Lisa
• Dennis/Belding
• Bayside High School
• Valley High School
  I repeat both Mario and Dennis have forgiven Dustin because they may noticed about what's wrong the entier original SBTB crewmember group, but it should be the bad relation between Peter Engel and the rest of the entire original SBTB crewmember group. As a result, those members of the rest of the original SBTB crewmember group should be blamed instead, TBH, IMO.

whats an albeist?
and whats peter engels relationship like with everyone?

Be prepared that both Mario and Dennis may have now noticed there's something wrong with the horrible relationship between Executive Producer Peter Engel and the rest of the original SBTB class crewmember group, just in case. Peter is not an ableist completely at all, but the other original SBTB class crew group members are considered ableists.

Also, he supposedly still keeps in touch with Dennis Haskins as well
and even Dennis was painted in a very unflattering light in Dustins book