Part of Kakashi's reputation was that he was a ruthless teacher at the academy; refusing to pass even one student because they didn't qualify as ninja in his eyes.
However, ever since Team 7 passed, I've not known Kakashi to personally fail any other teams.
Has Kakashi reversed course and passed every team that he's encountered since?
Kakashi did not fail any students after Team 7 in the original and Shippuden series, but he came very close in the Boruto series.
In the Boruto series, Kakashi offers to be the proctor during the graduation exam and the only student to pass is the one who takes the bell from him. Kakashi states this because he does not want an entire generation of students with defiant attitudes to become shinobi during the time of peace.
Iruka suggested that Kakashi not be too hard on the students come the final exams. Kakashi clarifies that even in peaceful times, they can't allow students with contemptuous attitudes to become shinobi, and as such will be as challenging as ever.
With that, Kakashi stated during the exams that only one student will pass - the others will fail.
During the practical test of the exams, the students were left awe-stricken that he was to be their head proctor. It was explained that while Shino Aburame, Anko Mitarashi, and Konohamaru Sarutobi would be evaluating the students' respective performances in a 24 hour field combat, ultimately only one student could pass; that student would be the one who takes the single bell from Kakashi's hip
However, much like the original series, they all learn the true lesson of the exam and Kakashi passes every student
Ultimately, time ran out before anyone could get the bell. Despite Boruto's plan failing, Kakashi passed everyone, noting that they succeeded in the true goal of the test: team-work and loyalty.
Kakashi became part of team 7 and so he could not take another team to teach until team 7 has become Jonins and would not need a teacher as much as before.
All the other teachers also take care of one team at a time (Guy, Minato, third Hokage...). It would be a rational procedure because we often see teachers accompany their disciples on missions and if they have several teams to look after, they would fail to take care of all of them properly.
As far as I know no.
He nearly failed Boruto in the Boruto anime though, which is considered canon.
Apart from that, I don't think he has gone about leading teams. I think team 7 was his only team. So to too with Kurenai, Asuma, and Gai.
But before team 7, the anime showed Kakashi failing 2 teams of 3 genins. But I am not sure if this is canon though as the anime tried to squeeze in as many episodes as possible.
