My understanding based on spiritual growth is that one needs to be guided by someone who understands the things of the spirit. The truth is that someone who is a teacher and someone who is a guru could be the right persons to go to for one to learn and acquire knowledge to grow spiritually in my opinion.
"Everything is created twice, first in the mind and then in reality" Robin Sharma
A guru is a mentor, master, or expert of certain knowledge of a field.
And, a teacher in a formal setting is also known as an educator who helps students to acquire knowledge or virtue, and informally, a teacher could be anyone.
This is the reason why I mentioned initially that either a teacher or a guru can be a go-to person for the aim of spiritual growth.
To find who we need to grow spiritually, it's important to understand why we need spiritual growth.
Some people don't believe in spirituality and it's okay to respect their belief, however, For those who believe in spirituality, spiritual growth is essential because it helps to bring pleasure to God. There is a statement that God is always there with and for us but the moment we stray away, it looks like he is far away, what happens at that point is that our sin makes us distant from him, the moment we repent of sin, God becomes happy and we feel his presence again.
Spiritual growth helps to draw us closer to God, it helps us to know God and understand his will. There are many temptations in the world, it takes the grace of God to overcome them and that is possible when God's word is being meditated day in day out.
Do we need a Guru or teacher to grow spiritually?"
We can have both so long they have a deep understand g of spirituality. As Christians, each teacher to church, we always listen to a sermon and teachings that edifies our so. Most of these pastors we listen to may not be a guru or teacher but they are still able to share the word with great understanding through the inspiration and interpretation from the rit and the constant reading of the word of God.