Honorifics for teachers

I'm an ALT at a high school and always use the honorific -sensei after the surname when I speak to my colleagues, and address the principal/vice principal as kochi and kyoto sensei. As the teachers at the school are of a higher rank than me should I use -senpai instead or is it fine just to use -sensei? I've also noticed some male teachers call the younger female teachers -san instead of -sensei, does this mean they have a sexist attitude towards young female teachers or is there another reason why they are not using the honorific -sensei?

You mean "kocho sensei" for principal, right? :)

From you to other teachers, who are more experienced than you are: "-sensei" would be fine.
(In high schools, students amongst themselves use "-senpai" to refer to students senior to them, but I have never heard this among teachers.)

You say "younger" female teachers, right? Then those male teachers consider them their "juniors," and prefer to call them as such, instead of adding the honorifics.

First of all, -sensei is the safest way you can call them, so I think you can stick to that.

Meanwhile as a parent, I've noticed that teachers often call each other -san when there are no students around. -San is a nuetral expression used among colleagues and business partners in Japan, so I suppose they use it when they don't have to be "role models" to the students.

I can't think of an example where calling people -san can imply sexist attitudes. -Chan can sound sexist depending on the situation/context, but -san is just a plain expression.

If you feel that those males call most females with -san, but only use -sensei to call males, perhaps it's because they feel they should keep a man-to-man relationship among the males rather than that they feel something special towards females. Like many Western countries, men often respect a jock-like relationship among "the guys."

You can call the other sensei as "san"(eg, Suzuki-san/sensei) if there are no pupils there such as in teacher's room. You are better to call as Suzuki-sensei while pupils are there.
The title of "Sensei" is used for people who teach you and the others, and also used for those whom they are at higher position and prestigious position(eg, medical doctors, priests, lawyers, and accountants).
I believe you are quite young teacher in the school, so it can be accepted to call the other colleagues as "sensei", this is because all other colleagues are older and experienced teachers than you.
Experienced teachers don't normally call "Sensei" to those young teachers . The title of "Sensei" at schools used to those who are older than you even though between teachers and teachers, so it sounds slightly strange that you call the principal as "san"(Suzuki san instead of Suzuki kocho/sensei).
Experienced male teachers normally call without any title to those who are young male teachers(eg, Suzuki). It like a relationship between male teachers and pupils, and still often male teachers call young female teachers without title while there are no pupils. This is because non-title teacher is young person.
Teaching position in JP is very high and respectable(not as higher as it used be), so all people must respect teachers even though they are young. Therefore, teachers don't call the other teachers as "san" while pupils are there.

"Senpai" is used mainly at schools between senior and junior students. They study and belong to sporting and cultural clubs at school in different age classes. They share the same experience there.
"Senpai" can be accepted to use at work places but I think it's minor situation these days. Work places such as Police or defense force can be accepted due to remaining classic hierarchy culture.

Thank you so much Uco and tokyo friend, this clarifies a lot! I always thought sensei was right but read somewhere that I should always call older/more experienced people senpai which seemed strange. It's good to hear the explanation for older teachers using -san too. I thought maybe it was a sign that they did not take younger teachers seriously as -sensei seems more respectful. When I go to the doctor I call him sensei and not by his surname, is that okay? Should I say things like "shitshurei shimasu" before I enter his room the way a student would before they enter a staff room? And when I leave the doctor's office should I say "otsu kara sama desu" or is it fine just to say "arigatou gozaimasu"?

When I go to the doctor I call him sensei and not by his surname, is that okay?

That's the most common way to address a doctor in person.

Should I say things like "shitshurei shimasu" before I enter his room the way a student would before they enter a staff room?

You can use that expression any time you enter somewhere. I often say that when going into the examination room, or I might just say "yoroshiku onegaishimasu." Or when you get to know that doctor well, you can get away with a "konnichiwa."

And when I leave the doctor's office should I say "otsu kara sama desu"

That's not commonly said. The most common phrase here is "arigatou gozaimashita."

Thanks Uco!