Is the use of future tense (especially "will" and "shall") going out of grammar?
Solution 1:
In the old days, shall was used with the first person and will was used with the second and third persons. This is no longer the case. Neither word is becoming extinct. In fact they are not even endangered.
Solution 2:
I am to go to London
has a special meaning, or range of meanings, which go beyond stating a simple future tense. Usually it means this:
A decision was made by someone else for me to go to London.
It is an example of the "to be" + infinitive construction, exemplified by clauss like "you are to be quiet" (to be + to be) or "you are to take your medicine twice a day" (to be + to take).
It indicates the requirement for someone to comply passively with some requirement that comes from source which is not given in that clause (but can be given in other clauses, for instance: "according to your doctor's prescription, you are to take this medicine twice a day".)
It can be used in conditional constructions:
If I am to go to London this time of year, I better pack a raincoat.
The helping verb "will" for indicating future actions is not falling out of use at all.
Your teacher is feeding you severe misinformation about the English language; find another.
Solution 3:
In place of a future tense, English can express the future in the following ways:
will (sometimes shall) + verb
be + going to + verb
present tense
present progressive construction
will + be + -ing form of the verb
be + infinitive, as your teacher suggested, normally occurs only when it is necessry to convey a degree of obligation.
Your sentence ‘Freedom is my birthright and I shall have it’ is perfectly grammatical, but, as others have said, shall is no longer very much used.
Solution 4:
Perhaps your teacher means "there is no such thing as an inflected form for the future tense". This is true for English, whereas many latinate languages have an inflected form for the future (Spanish: 'comer', to eat, 'comeré', I will eat [future]).
"There is most certainly a method of referring to actions which will (future) occur in the future; I have just used it."
Our only requirement is that we use an auxiliary/modal verb to accomplish this (combining tense+aspect). Just a few searches about the history/etymology of English have helped me to guess at what your teacher might have meant--perhaps if it is for a class the whole purpose of the task at hand is to utilize the Present Indefinite. I am to [verb] + [action], sounds like circumlocution--it might be able to logically replace 'shall' in the sentence you described but it will leave the reader asking "why on Earth has this idiot not simply used shall or will?". I use will every day and will continue to do so until the day I die, whether it be a true future tense or no.
Solution 5:
Your question conflates at least two issues:
(1) is will (and shall) going out of use in English?
(2) either way, does English have such a thing as a "future tense", and if so, is this what is represented by will/shall?
In answer to (1), will and shall are very much in use, but, like other options for expressing "futurity", they don't simply express that notion and nothing else, and so aren't always the most idiomatic option. For example, in the 1st person, they typically express a more instant decision, whereas "is/will be ...ing" expresses a planned action. In all persons, they can also often express a formal, planned action as part of a timetable, as opposed to a more informal arrangement that might be expressed with "is ...ing" etc.
Now, as to whether English has a "future tense", this really depends on your model/analysis. If you see "tense" as being any construction that grammaticalises time, then you may well decide that will/shall can be a grammaticalisation of time, and that they should be included in the category "tense". Will/shall are also surely grammaticalisations of other notions, but on the other hand, so are the things that we often label as "future tense" in languages generally.
On the other hand, if you take the view that takes~took is what constitutes "tense" in English, there is an argument that this opposition clearly operates on a different "dimension", or belongs to a different system, compared to the opposition will/shall and other modals.
As usual, which is the "right" answer is more a question of the purpose of your analysis. If it's essentially a labelling/stamp-collecting issue, then whether to apply the label "tense" or not is largely arbitrary...