Identity versus location
是 links a subject to its category, name, or description. 他是医生 (he is a doctor), 这是我的书 (this is my book). 在 places a subject at a location or inside a state. 他在医院 (he is at the hospital), 书在桌子上 (the book is on the table). The rule is simple: if the English "is" is followed by a thing or concept, use 是; if it is followed by a place or location, use 在.
The common beginner error
English "she is in Beijing" gets translated as 她是在北京, which is wrong. Correct Chinese is 她在北京 (she is at Beijing). The 是 is the wrong verb here because "in Beijing" is a location, not a category. Similarly, "the book is on the table" becomes 书在桌子上, never 书是在桌子上. Mastering the split is one of the first hurdles in Mandarin; it does not exist in European languages.
他是老师,现在在学校。
Tā shì lǎoshī, xiànzài zài xuéxiào.
He is a teacher, and right now he is at the school.
在 also marks ongoing action
Besides location, 在 marks progressive aspect: 我在吃饭 (I am eating). This use comes before the verb: Subject + 在 + Verb. It is the Mandarin equivalent of English "-ing". 是 never plays this role; 我是吃饭 is nonsense. Differentiate by what follows 在: a noun/place = location 在; a verb = progressive 在.
我在家在看电视。
Wǒ zài jiā zài kàn diànshì.
I am at home watching TV.
Emphatic 是 before 在
One pattern combines both: 是 + 在 + location + Verb emphasizes the location. 我是在北京学的中文 (it was in Beijing that I studied Chinese). Here 是 is an emphatic copula pointing at the location; 在 still marks the location itself. This is the 是...的 emphasis structure applied to locations. Unless you are using the 是...的 pattern deliberately, do not combine the two.