If the teacher really wants to refer to the food substance then the correct term is "banana" not "bananas", as already noted in an earlier answer:
Example of use -- this nutritional article does not follow a uniform style in using 'banana/bananas' but these extracts represent correct usage when referring to the food substance:
Compared to apple, banana has four times the protein, twice the carbohydrates (...) Banana is rich in vitamins B6 and B12
Source: Health Benefits of Bananas
If "bananas" is being used as a generic name for a type of fruit, then it is an English convention of usage that when plural words like"apples", "oranges" or "bananas" are used as a generic name, the word is followed by the plural form of a verb; so the plural "are" is typically used rather than the singular "is":
Apples are nutritious.
Oranges are rich in vitamin C.
Bananas contain (not contains) calcium and vitamins.
Bananas are nutritious.
If you are particular to combine 'banana' with 'sweet fruit' then typical constructions would be
The banana is a sweet fruit.
Bananas are sweet fruits.
Note that both 'the banana' and 'bananas' are used as generic names here.
More complicated for learners:
Bananas are a sweet fruit.
Bananas are sweet fruit.
Here "fruit" is used as plural instead of "fruits", or to describe "a type of fruit", and "bananas" is also used as a generic name for a type of fruit, so we are really saying either that bananas are sweet fruits, or that bananas are a type of sweet fruit. This usage appears to be more common in the USA. See this comment posted 2 days ago, and this relevant answer on English.SE: https://english.stackexchange.com/a/403172/231519
You don't see "bananas is" used anywhere in this context. It is not grammatical to say "bananas is sweet fruit" or "bananas is a (type of) sweet fruit." So the teacher is mistaken here. Unfortunately he might have learned his English by rules rather than practice, and like many another English Language Learner, got confused by a notoriously tricky language!
However you cannot argue so easily with a teacher in Asia or the Middle East. See my closely related question, and another member's question about your exact situation, at Interpersonal Skills Stack Exchange, for some interpersonal solutions:
How can I get more co-operation from a teacher who discourages a student from asking questions to clear his confusion?
How to politely correct a teacher?
Sad but true: "bananas" when used as idiom has some other meanings that the teacher probably wouldn't like to hear...
Considering your titular question here:
Is "bananas is" possible by any chance?
If the question does not restrict answers to the "sweet fruit" part then it is very possible to write "bananas is" in a simple sentence as in
bananas is the go-to food for athletes to quickly regain energy and potassium during tiring games.
Here "bananas" is a food choice and linked with "the go-to food": therefore we use the singular form "is".
Also, if the use of "bananas" is not restricted to a generic name for a type of fruit:
three bananas is a substantial breakfast.
Here "three bananas" describes a food unit and can therefore be taken as a single entity, so 'is' sounds better than 'are', especially when linked to "a substantial breakfast" (the construction "three bananas are a substantial breakfast" might be technically correct but doesn't sound so right to my ear. I am curious to know what native speakers feel about the natural choice of is/are here.)