Question: Isn't one tie allowed in first species if the CF is shorter than 10 notes?
Again, I don't want to drill 'rules' into your head, so let me give you an answer more meaningful than "don't break the rules!" The important thing is to recognize the effect of a tie in such a context as our short exercises. It will create an accent simply because it differs from the rest of the exercise in a very noticeable way. So, if you do want to use a tie, make sure that you use it in a musically appropriate place (e.g. a suspension at a climax).
Top Voice: E G A B D C A G# A
Interval: 5 3 3 5 8 6 6 6 8
CF: A E F E D E C B A
Nice work! Your line has a very nice shape to it, and you change harmonic 'colour' on the climax (an octave D). Also, your use of the modal G at the beginning allows the G# at the end to create a new harmonic colour at a significant point in the phrase (the cadence).
I am going to make one suggestion, not because there is particulalry anything 'wrong' with the line, but because it is something I want you to think about. At the opening of the line (not a very significant section) you have both voices leaping in the same direction from an open fifth. Although this is not really bad in itself, the fact that you are leaping to the leading tone in one of the voices creates a very strong 'accent'. Is this really an appropriate place for such a thing? In any case, I still think your line is very good...but what kind of a teacher would I be if I let even the littlest thing slide

.
Here are a few suggestions:
A G A C D B A G# A - Here we avoid the leap at the beginning, and approach the climax by step. This gives the listener a sense of preparation for the climax ( warning per se). This is not necessarily better, but you should be aware of the difference between leaping to a climax and approaching one stepwise. Note that the line is stuck on the A, so...
A G A C D B A B C - As in the previous line, the (E-B) 5th seems better suited after the climax...it doesn't give away the 'open' colour you have reserved for the climax before it happens. Also, to avoid becoming stuck on the A, the soprano climbs to scale degree three. Here we sacrifice the new colour of the G# at the cadence, but we add something else by not ending on the tonic.
Anyway, these lines aren't necessarily better than yours, but they do demonstrate some other valuable tools that can be used when writing counterpoint.