How long/how many pages does a teen science fiction novel need to be in order for a publishing agency to take it seriously?
I will answer your first question, as it seems to me like you are asking two questions. How long should a sci-fi Young Adult Novel be?
First of all, the number of pages in a book does not matter. Adjusting the margins, font size, or spacing even slightly could alter the page count. What matters is the word count.
A novel is classified as any book with 40,000 or more words. For adults, about 80,000 seems to be the sweet spot. However, Sci-fi novels tend to be longer, averaging 100,000 to 115,000 words in length.
On the other hand, Young Adult novels are usually between 60,000 and 80,000 words, probably due to shorter attention spans or something else about the target audience. Balancing these two figures is also a question I struggle with.
I would say that once you hit about 50,000 you should be good, as in long enough that publishers would "take you seriously". However, it would be a pretty short novel. If you are aiming to be average-sized, I would suggest that your novel be in the range of 75,000 to 90,000, because of the average longer length of sci-fi novels in general.
Publishers take a lot more into consideration when publishing a novel, and word count ends up being one of the smaller factors when publishers decide if they want to publish your novel. Factors like writing quality are immensely more important, so remember that. If you're writing is stellar, your characters deep, your plot intricate, your world developed, then publishers will usually be willing to forgive any grievances on the word count front. Unless, of course, you're novel is really short. Again, I would caution against going below 50,000 or 55,000 words.