According to the History teacher in my school, Indra can grant moksha according to the Vedas. Now this seems very appalling to me since Indra is not the supreme God. So is she right?
I'm giving answer from Advaita perspective.
Anyone who has Brahma-Jnana can liberate another by bestowing Brahma-Jnana to him.
Is it supported by scriptures?
Yes, Mundaka Upanishad says that one can adore a person who know self to get any worldly desires if he has worldly desires:
III-i-10: The man of pure mind wins those worlds which he mentally wishes for and those enjoyable things which he covets. Therefore one, desirous of prosperity, should adore the knower of the Self.
Thus one who has desires to gain worldly gains can worship a Brahma-Jnani and he can get those prosperities. But if the person has no desire for worldly gains then he can worship that Jnani for liberation:
III-ii-1: He knows this supreme abode, this Brahman, in which is placed the Universe and which shines holy. Those wise ones indeed, who having become desireless, worship this (enlightened) person, transcend this human seed.
So, if one worships a Jnani with a desire for liberation, he can get liberation too.
Whether Indra has Brahma Jnana or not?
As clearly seen from Kena Upanishad episode Devi Uma gives Brahma Jnana to Indra:
IV-1. She said: “It was Brahman. In the victory that was Brahman’s you were revelling in joy”. Then alone did Indra know for certain that It was Brahman.
IV-2. Therefore, these gods viz. Agni, Vayu and Indra excelled other gods, for they touched Brahman who stood very close and indeed knew first that It was Brahman.
IV-3. Therefore is Indra more excellent than the other gods, for he touched Brahman who stood very close and indeed knew first that It was Brahman.
Thus, it is clear that Indra has Brahma-Jnana so if one worships Indra with desire to get liberation, Indra can liberate him.
In the commentary of Mundaka Upanishad, Adi Shankara states various ways to worship such Jnani:
by devout washing of his feet, by serving him and prostrating, etc. to him.
Are there any examples?
Yes, as seen in Kaushtiki Brahmana Upanishad which I also discuss here, Indra desires to give Brahma Jnana to Pratardana;
III-1. Pratardana, the son of Divodasa, by means of fighting and virility, verily reached the beloved abode of Indra. To him then Indra Said: Pratardana, choose a boon. Then said Pratardana: Do you yourself choose that boon for me which you deem most beneficent for man. To him then Indra said: A superior verily chooses not for an inferior. Do you yourself choose. ‘No boon verily then is it to me’ said Pratardana. But Indra did not depart from the truth, for Indra is truth. To him then Indra said: ‘Understand me only. This indeed I deem most beneficent to man, namely that one should understand me. I slew the three-headed Tvastir; I delivered the Arunmukhas, the ascetics, to the wolves. Transgressing many compacts I killed the people of Prahlada in the sky, the Paulomas in the atmosphere, the Kalakanjas on the earth. Of me, such as I was then, not a single hair was injured. So he knows me thus – by no deed whatever of his is his world injured, not by stealing, not by killing an embryo, not by the murder of his mother, not by the murder of his father. If he has done any evil, the dark colour departs not from his face.
III-2. Then he (Indra) said: I am the Spirit of the vital breath, the intelligent Self. As such, worship me as life, as immortality. Life is the vital breath: the vital breath is life. For as long as the vital breath remains in the body so long is there life. For indeed with the vital breath one obtains immortality in this world; with intelligence, true conception. So he who worships me as life, as immortality, reaches the full term of life in this world; he obtains immortality and indestructibility in the heavenly world.
Thus if you worship Indra with desire to get worldly prosperity then you can get it and if you worship Indra with desire to get liberation you can too get it.