Praise be to Allaah.
What your muallim (teacher) has told you, that in sujood the toes should be bent and should touch the ground, is correct. It is a proven part of the sunnah of the Prophet (peace and blessings of Allaah be upon him). Al-Bukhaari (may Allaah have mercy on him) reported in his Saheeh from Abu Humayd al-Saaidi that the Prophet (peace and blessings of Allaah be upon him) when he prostrated the tips of his toes would be facing the qiblah (Fath, no. 828).
Abu Dawood reported from Abu Humayd al-Saaidi: he would bend his toes when he prostrated. (Reported by Abu Dawood, no. 716, Bab fi iftitaah al-salaah). The author of Awn al-Mabood Sharh Sunan Abi Dawood said: He would bend (yaftukh) his toes the meaning of fatkh is to bend or flex, so he would bend his toes and point them towards the qiblah. (Awn al-Mabood, 2/419). The hadeeth was reported by al-Albaani in Sifat al-Salaah p. 142, 1411 AH edition [This book is also available in English under the title The Prophets Prayer Described Translator].
Pointing the toes towards the qiblah is sunnah and is encouraged, but if a person does not do it, his prayer or leadership of the prayer is still valid, especially as many people do not have the ability to flex their toes in the manner described. But everyone should put their toes on the ground when prostrating, because they are counted among the seven parts of the body on which we are commanded to prostrate. If a person lifts his feet off the ground when prostrating, and does not place the tips of his toes on the ground, then his prayer or leadership of the prayer is not valid.
With regard to the way one should sit between sujoods, two ways of doing this have been narrated from the Prophet (peace and blessings of Allaah be upon him).
Keeping the right foot upright and sitting on the left foot, as was described in the saheeh hadeeth which describes how he sat between two sujoods: He kept his right foot upright, and he pointed its toes towards the qiblah. This is the most well known way, and it is the way he sat most of the time.
Keeping both feet upright and sitting on the heels, as was described in the saheeh hadeeth which describes how he sat between two sujoods: Sometimes he would rest on both his heels and the balls on his feet. (See Sifat al-Salaah by al-Albaani, p. 152). This is not the way of sitting that is forbidden.
If a person who is praying puts one foot on top of the other and sits on them, he is doing something other than the sunnah, but his prayer or leadership of the prayer is still valid. In all cases, we should guide people towards the sunnah and teach them about it, because this is an important matter how could it be otherwise? The Prophet (peace and blessings of Allaah be upon him) said: Pray as you have seen me praying. (Reported by al-Bukhaari, no. 631). And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid