Let us assume Priya's rank from the top is \( P \) and Charan's rank from the top is \( C \).
We are given that there are six students between Priya and Charan, which means the difference in their ranks is 7. Hence, we can write:
\[
|P - C| = 7
\]
From condition II, Charan's rank is 8th from the top:
\[
C = 8
\]
Now, using the equation \( |P - 8| = 7 \), we can solve for Priya's rank \( P \):
\[
P - 8 = 7 \quad \text{or} \quad 8 - P = 7
\]
Solving the first equation:
\[
P = 15
\]
Solving the second equation:
\[
P = 1
\]
So, Priya can either be 1st or 15th. Since there are 70 students in the class, Priya's rank from the top is the 15th position.
Thus, Priya's rank is \( \boxed{15} \).