The tenses in
V1て+V2 depend on what V1 and V2 are.
When V1 and V2 are both just normal verbs, such as:
John-TOP run-TE ate-PAST
"John ran and ate."
the tense of V2 applies to V1 as well.
V2 modifying V1
There are some V2s, such as いる、おく、しまう、もらう、くれる、あげる、ほしい which simply modify V1. (You can detect that you are working with one of these because you cannot insert adverbs after V1て.) These cases require more care.
phone-number-OBJ teach-TE want-PAST
"I wanted you to tell me the phone number."
Here, 教えて has no tense (and neither does "tell" in English).
For your specific example:
students-IOBJ math-OBJ teach-TE give-PAST
"I taught the students math for their benefit."
As you can see in the English, changing the tense of
V1てあげる ("doing V1 for someone's benefit"), clearly changes the tense of V1.
You can basically intuitively tell how tense markings affect V1 once you learn the meanings of the various V2s which fall into this category.