honeyche,
considering your inquiries, first things first, did the school provide you with any document about your perfomance? a sort of performance evaluation to apprise you on how you did in school, the manner you comply with the set standards of the school. This could be the basis of the school on how you perform and can be used against you in the event that you did not conform with their set standards.
In the event that none was given to you, they can not just text you and say that they will no longer renew your contract on the basis of of an SMS. It is quite sad knowing that your contract will no longer be renewed but on normal circumstances a contract do have stipulations clearly written and shall be followed by both parties. In your case, if the contract expires and the party involved cease to continue the agreement, it shall have a binding effect.
However, in your case, you are under the so-called termed employee. There are relevant jurisprudence decided by the Supreme COurt, Brent School vs. NLRC, you can find it sa google and try to read it. To me, your case squarely fit this case.
Now, filing a case of illegal dismissal before the NLRC is always your contitutional right under the law and your statutory right to due process. I would suggest that before instituting a case before the NLRC, try to gather as much documentary evidence as possible. Always remember, filing a labor case should be backed with documentary evidence to show the arbiter that your cause of action is valid. These cases are ruled based on the preponderance of evidence. The more evidence you have more chances of you winning your case.